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Teacher’s ManualScience OrbitThe Chemistry

(An imprint of New Saraswati House (India) Pvt. Ltd.)New Delhi-110002 (INDIA)

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(An imprint of New Saraswati House (India) Pvt. Ltd.)

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First published 2018

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The purpose of The Science Orbit series (3 to 8) teacher’s resource pack is to empower teachers to make classroom teaching a holistic experience. It will enable teachers to explain the chapters in the most effective way, which will not only impart knowledge, but also ignite interest in the minds of young learners towards the subject. A wide array of resources complement these manuals, which makes learning an interesting process instead of a routine chore.

Teacher’s Manual

Teacher’s Manual resources have been carefully prepared with an aim to make the process of teaching and learning interesting and intriguing for the teachers and learners alike. It comprises detailed lesson plans and answers to the coursebook along with solved worksheets and model test papers. These have been prepared keeping in mind the explanation of the concepts and the level appropriateness of the topics. Easily available teaching aids are used to make teaching and learning an interactive and lucid process. The lesson plans give a topic-wise explanation of each chapter. Its components are:

– Warm-up section guides the teacher to start the topic in an interesting way.

– Learning objectives give the list of measurable aims of each chapter, which should be achieved after teaching the chapter.

– Concept explanation gives a detailed method of explaining the important concepts of the chapter using various teaching aids.

– Reinforce section allows the teacher to check the progress of the concepts learned by the students with the help of textual questions and worksheets and allows them to revisit and revise the concepts, if required.

– Explore section helps the learners to do various activities, often taking them beyond their classroom learning.

App-based Digital Store

Digital offerings comprise digital resource embedded e-book, 2D and 3D animations, videos, interactive activities, slideshows, educational games, test generator and teacher’s manual.

Web Support

The web support consists of worksheets, model test papers, and answers to worksheets and model test papers. These would help teachers in assessing students on the concepts taught in the class.

Preface

Contents

1. Matter and its Composition 5

2. Physical and Chemical Changes 13

3. Elements, Compounds and Mixtures 22

4. Atomic Structure 32

Model Test Paper 1 42

5. Language of Chemistry 44

6. Metals and Non-metals 51

7. Air and Atmosphere 60

Model Test Paper 2 72

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Learning Objectives

Students will be able to: understand matter and its composition

know about matter and its states

understand the concept of elements and compounds

differentiate between different states of matter on the basis of interparticle space and force between them

know about the composition of different kinds of matter

1 Matter and its Composition

Warm-upAsk the students about the things around them. Everything around us is made up of matter. Air occupies the entire atmosphere, though it is not visible. Tell them that air, stone and water are different forms of matter. Explain them matter is made of small building blocks called molecules and the molecules are made of atoms. But, molecules are also the smallest particles of matter that can exist independently.

Concept Explanation

Matter and its StatesExplain to the students that everything around us is made up of matter. Also state that anything that occupies space and has mass is called matter. In nature, matter exists in three forms: solid, liquid and gas.

Explain to the students that air occupies space by a simple experiment.

Take a syringe and pull the piston so that air fills the syringe tube. Seal the mouth of the

syringe with the help of clay. Now press the piston. Students will observe that the piston cannot be pressed now as air has filled the space of the syringe tube. This shows that air occupies space.

Solid, Liquid and GasExplain to the students that solids have definite shape and a fixed volume, and can be rigid or flexible, hard or soft, compressible or non-compressible and light or heavy. Tell students to prepare a list of the solids they see around them.

Next, explain to them about liquids which have no defi nite shape but a fixed volume. Liquids take the shape of the container they are poured.

Now explain to the students about the gases which neither have a definite shape nor a definite volume. Gases cannot be confined to an open container.

Liquids and gases are also called fluids as they can flow.

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Composition of MatterExplain to students that matter is composed of either a pure substance or a combination of two or more pure substances. A pure substance has fixed chemical properties and physical properties. It can either be an element or a compound.

Elements are made up of smaller particles called atoms. For example, hydrogen, carbon. Atoms of certain elements join to form larger particles called molecules. A molecule is the smallest particle of an element or a compound, which is capable of independent existence. For example, a hydrogen molecule is made up of two hydrogen atoms.

Compounds are pure substances composed of two or more different elements that combine chemically. The properties of a compound are always different from those of the elements found in the compound. Explain to the students by taking an example such as water.

To reinforce the concepts learnt before ask the students to do the related Pause and Do section from the textbook.

Forces that keep the constituents of matter togetherExplain to the students the Kinetic Theory of Matter (The Particle theory) which states that all matter consists of many small particles. Tell them about the reasons for the difference in the arrangement of particles (atoms or molecules) which include interparticle space,

interparticle force and Brownian movement among particles.

In solids, molecules are very closely packed, intermolecular attraction acts as a cohesive force and holds the molecules together.

In liquids, molecules are not so closely packed. Intermolecular force in molecules is much less than that in solids. Liquids are able to move around in all directions but their movement is limited.

In gases, molecules are not packed. Intermolecular attraction in gases is negligible. Therefore, molecules can spread over the entire space available.

Differences between the three states of matterExplain to the students the differences between the solid, liquid and gas (Refer to table on page 13 in the textbook).

ReinforceTo reinforce, ask the students to read the Summary and Glossary sections and do the Exercises and Learning Zone sections from the textbook. Help students develop skills (research and thinking) given in the Skills for Life section in the chapter.

ExploreAsk students to do the Learn by Doing section given at the end of the chapter.

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Worksheet 1

1. Name the following.

a. The smallest particle of any compound __________________

b. Attractive force between the molecules __________________

c. The amount of matter that a body has __________________

d. State of matter with defi nite shape and fi xed volume __________________

e. An example of fl uids __________________

f. An example of solid that can compress __________________

g. Movement of particles __________________

2. Complete the following table.

Property Solid Liquid Gas

Ability to fl ow ________________ ________________ Flow randomly in every direction

Rigidity Hard, strong ________________ ________________

Intermolecular force of attraction ________________ Weaker than solids ________________

Volume ________________ ________________ ________________

3. Which state of matter fl ows in all the directions? Why?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

4. What are pure substances?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

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Worksheet 2

1. Name two elements that do not have the ability to form molecules and exist as single atoms.

________________________________________________________________________

2. We can pour sand into a cup but it isn’t a liquid. Explain.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

3. Show with the help of diagram the intermolecular spaces between solids, liquids and gases.

4. Why do various materials differ in their properties and behaviour?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

5. Give an example of each.

a. A heavy solid ____________________

b. A light solid ____________________

c. A hard solid ____________________

d. A soft solid ____________________

e. A rigid solid ____________________

f. A fl exible solid ____________________

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Answers to Coursebook

PART IA 1. (a) 2. (b) 3. (c) 4. (b) 5. (a) 6. (d)B 1. mass 2. 8 3. chemical 4. gases 5. solidC 1. False 2. True 3. False 4. True 5. True 6. False 7. FalseD 1. Mass 2. Pressure or force or heat 3. Element 4. Fluids 5. Interparticle space 6. Interparticle force

PART IIA 1. The fi ve states of matter are solid, liquid,

gas, plasma and Bose-Einstein condensate. 2. Helium, argon 3. (a) Solids: wood and sand (b) Liquids: milk and oil (c) Gases: oxygen and hydrogen (d) Elements: hydrogen and carbon (e) Compounds: water (H2O) and carbon

dioxide (CO2)B 1. Matter is anything that occupies space

and has mass. For example, wood, air and water

2. (a) Solids: (i) A solid has a definite shape and a

fixed volume. (ii) They retain their shape unless they

are acted upon by an external agent such as pressure, force and heat.

(b) Liquids: (i) A liquid has a fixed volume, but it

does not have a fixed shape. (ii) Liquids take the shape of the

container they are poured into. (c) Gases: (i) A gas has neither a definite shape

nor a definite volume. (ii) Gases cannot be confined in an

open container.

3. An element is the simplest form of matter that cannot be further broken down using any chemical means. Elements are made up of smaller particles called atoms. Hydrogen, nitrogen, carbon, oxygen, potassium are the examples of elements.

4. A pure substance has a fixed chemical composition throughout. It has well-defined physical properties, such as colour, taste, state, smell, melting point, boiling point, etc.

5. A molecule is the smallest particle of an element or a compound, which is capable of independent existence. For example, a hydrogen molecule is made up of two hydrogen atoms.

6. In solids, molecules are very closely packed such that there is no space between them. Therefore, they are not able to move.

C 1. In solids, molecules are very closely packed such that there is no space between them. Therefore, they are not able to move. Moreover, intermolecular attraction acts as a cohesive force and holds the molecules together. A solid has a definite shape and a fixed volume. A wooden block, a piece of sponge, sand are the examples of solids.

In liquids, molecules are not so closely packed. There is some space between them such that they can move about. Intermolecular force between the molecules is much less than that in solids. Liquids are able to move around in all directions but their movement is limited. A liquid has a fixed volume, but it does not have a fixed shape. Liquids take the shape of the container they are poured into. Water, juice, milk, oil, are some examples of liquids.

In gases, molecules are not packed. They are free to move. Intermolecular attraction in gases is negligible. Therefore, molecules can spread over the entire space available. As a result, they do not have a fixed shape

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or a fixed volume. Air, carbon dioxide, nitrogen, steam, hydrogen, etc. are some examples of gases.

2. A hydrogen molecule is made up of 2 hydrogen atoms.

Hydrogen Hydrogen

A sulphur molecule is made up of 8 sulphur atoms in a ring form.

(Refer diagram fi g.1.4 for sulphur)

3. To show that air occupies space take a syringe and pull the piston so that air fills the syringe tube. Seal the mouth of the syringe with the help of clay. Now try to press the piston. We observe that the piston cannot be pressed now as air has filled the space of the syringe tube. This shows that air occupies space.

(Refer experiment on page 12)

4. Refer to the table of differences between solid, liquid and gases on page 13.

5. (a) Solids and liquids have a fi xed volume, whereas gases do not have a fi xed volume.

(b) Solids have a fi xed shape whereas liquids and gases do not have a fi xed shape.

(c) Liquids and gases fl ow but solids do not fl ow.

(d) Solids may be compressible or non-compressible. Liquids are slightly compressible whereas gases are highly compressible.

6. Refer fi gure 1.6

7. Fluids are the substances, as a liquid or gas, that is capable of fl owing. Water, smoke, oil, water vapour, air are fl uids.

D 1. Gases cannot be confined to an open container as most of the gases cannot be

seen with the naked eye. Gas molecules are not packed and so molecules can spread over the entire space available.

2. Air is a mixture because different gases mix together to form air and no new compound is formed. The components of air change from place to place whereas that of water remains the same. Water is a chemical compound made up of hydrogen and oxygen combined in the ratio of 2 : 1 by volume.

Different gases can be separated from air through physical process whereas compounds are formed by chemical combinations and can only be separated by chemical methods.

3. In solids, molecules are very closely packed. Therefore, they are not able to move. The intermolecular attraction acts as a cohesive force and holds the molecules together. Thus, solids cannot be compressed easily.

4. In gases, molecules are not packed. Intermolecular attraction in gases is negligible. Therefore, molecules can spread over the entire space available and fl ow in any direction.

E 1. The tiny particle of a chemical element, which may or may not exist independently is called an atom. Molecules refers to the set of atoms held together by bond, indicating the smallest unit of a compound.

2. A solid has a defi nite shape and a fixed volume whereas fl uids can be either liquid or gases. A liquid has a fixed volume, but it does not have a fixed shape. A gas has neither a definite shape nor a definite volume. Solids molecules cannot move whereas in fl uids molecules can move.

3. An element is the simplest form of matter in that it cannot be further broken down using any chemical means. Elements are made up of smaller particles called atoms.

Compounds are pure substances composed of two or more different elements that combine chemically. The properties of a compound are always different from those

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of the elements found in the compound.

4. Mobility refers to the movement of the particles/molecules. In solids, the molecules cannot move. Liquids can fl ow from a higher level to a lower level and gases can fl ow in every direction.

Compressibility basically means how much we can press them. This depends on the arrangement of molecules. Solids and liquids have very little free space between particles. So they are not easily compressible. Gases, on the other hand, are easily compressible as they have a lot of free space between particles.

5. Interparticle space is the vacant space between the molecules. It is very less in case of solids and maximum in case of the gases. The interparticle force is the attractive force between the molecules. It is the most in case of solids and least in the case of gases.

LEARNING ZONE

Picture based questions

1. In the experiment, air has fi lled the whole space of the glass and when tilted, the air escapes out from the glass in the form of bubbles. This proves that air occupies space.

2. (a) (i) water (H2O)

(ii) sulphur dioxide (SO2)

(iii) oxygen molecule (O2)

(iv) carbon dioxide (CO2)

(b) (i)

(ii)

(iii)

(iv)

3. (a) solid (b) liquid and gas (c) solid

(d) gaseous state (e) gas

Think and answer

Matter can change from one form to the other. Water exists in three states of matter. Ice is a solid, water is a liquid and steam is a gas. Water can change its state when it is heated or cooled. When ice is heated, it changes into the liquid form and then due to further heating, it changes from liquid to steam or water vapour. On being cooled, steam changes into a liquid and the liquid on further cooling changes into a solid. The water cycle in nature is the effect of the evaporation of water and the condensation of water vapour back into water.

Answers to WorksheetsWORKSHEET 1

1. a. molecule b. interparticle force c. mass d. solid

e. oil f. foam g. Brownian movement

2.

Property Solid Liquid GasAbility to fl ow Do not fl ow Flow easily Flow randomly in

every directionRigidity Hard, strong Not rigid Not rigidIntermolecular force of attraction

Very high Weaker than solids Negligible

Volume Fixed volume Fixed volume Does not have a fi xed volume

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3. Gas, because the particles of a gas are not packed and they are free to move at will.

4. A pure substance has a fixed chemical composition throughout. It has well-defined physical properties, such as colour, taste, state, smell, melting point, boiling point, etc.

WORKSHEET 2

1. Helium and argon

2. Sand is a collection of very tiny solid particles. Each particle of sand has a defi nite shape. It can be poured due to small size of the particles, but comes in the category of solids.

3. Refer fi gure 1.6

4. The differences in the properties and behaviour of materials is due to the difference in the arrangement of particles (atoms or molecules) which is due to the following reasons: inter particle space, inter particle force and Brownian movement among particles.

5. (a) a metal sphere

(b) a piece of paper

(c) a steel rod

(d) cotton

(e) a wooden block

(f) a rubber band

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Learning Objectives

Students will be able to: differentiate between the different types of changes

understand why certain changes are desirable, periodic, reversible and others are undesirable, non-periodic and irreversible

differentiate between exothermic and endothermic reactions

know about the different types of physical changes like evaporation, sublimation, melting, freezing, boiling and condensation

understand the factors that affect evaporation

learn the various process involved in chemical changes like rusting, burning, fermentation, respiration and digestion

2 Physical and Chemical Changes

Warm-upStart the chapter by interacting with students regarding different changes that frequently occur around them, be it in the school, in the neighbourhood and in the environment. Tell them to make a list of these changes and then make them analyse whether the listed changes can be reversed or not or a new substance is formed in the change.

Now, ask the students to complete the exercise given in the Warm-up section.

Concept Explanation

Types of changes

Physical and Chemical ChangesExplain to the students that a change is the transformation of a substance from one form to another. A change in which the chemical composition of a substance remains the same is called a physical change whereas a

change in which the chemical composition of a substance changes is called a chemical change. These changes can be reversible or irreversible, desirable or undesirable, exothermic or endothermic and periodic or non-periodic. Activity – Ask students to cut a slice of apple and brinjal and keep it exposed for some time. Ask them to observe. The colour of the cut surface changes due to the formation of new substance. This occurs due to the chemical reactions that occur between air and the enzymes in the cut surface of apple and brinjal.

Desirable and Undesirable ChangesExplain to the students that some of the changes that we see around us can be desirable such as growth of plants. These are termed as desirable changes. Tell them changes that bring about destruction or that are harmful for living things such as cutting

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of trees, rotting of fruits, etc., are called undesirable changes.

Periodic and Non-periodic ChangesExplain to the students that the changes that occur repeatedly after regular intervals of time are called periodic changes. Tell them changes that do not repeat themselves at regular intervals of time are called non-periodic changes. Give some examples of periodic and non-periodic changes.

Reversible and Irreversible ChangesGo through the examples of reversible and irreversible changes (either from the book or if a different example is there that has been suggested by the students at the beginning of the chapter) and explain to them that there are some changes that can be brought back to their pre-change or previous state which are called reversible changes. Some changes that cannot be brought back to their pre-change form are called irreversible changes.

Exothermic and Endothermic Changes Explain exothermic and endothermic changes with the help of some suitable examples. To reinforce the concepts learnt before ask the students to do the related Pause and Do section from the textbook.

Categorising Physical ChangesExplain to the students evaporation is constantly occurring in nature. It is a slow, reversible, desirable change and an endothermic process as water absorbs heat energy to change into vapour. Also make them aware of factors affecting evaporation and list the difference between evaporation and boiling. Explain to the students the type of changes involved in the conversion of different states of matter i.e. freezing, melting, condensation and sublimation. Explain to the students the process of sublimation with the help of ammonium chloride.


Categorising Chemical ChangesMake students aware of the following different types of chemical changes-- Fermentation: Explain to the students that this chemical change is brought about by enzymes. It is irreversible and endothermic but can be desirable or undesirable. Rusting: A chemical change that usually occurs when iron is exposed to oxygen and moisture in air. It is undesirable, irreversible and endothermic. Respiration: Respiration is a desirable, irreversible and exothermic change. Curdling of Milk: The process by which a liquid changes into a soft, semi-solid mass is called curdling. It is an irreversible, desirable or undesirable change. Burning or Combustion is a chemical change in which a substance combines with oxygen releasing light and heat. So it is a chemical change that is desirable, irreversible and exothermic. Cooking of food and coagulation of egg are examples of endothermic, fast and desirable changes. Digestion is a desirable, irreversible and endothermic chemical change.

ReinforceTo reinforce, ask the students to read the Summary and Glossary sections and do the Exercises and Learning Zone section from the textbook. Help students develop skills (research and thinking) given in the Skills for Life section in the chapter.

ExploreAsk the students to do the Learn by Doing section given at the end of the chapter.

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Worksheet 1

1. Fill in the blanks.

a. Changes that lead to the formation of new substances are called ______________.

b. Melting of candle wax is a ______________ change but its burning is a ______________ change.

c. Growth of a plant is a ______________ change while formation of clouds is a ______________ change.

d. A brown layer formed when an iron article is left exposed in air in an open area is called ______________.

e. Digestion is a chemical change which is _________, _________ and _________.

f. The fermentation of fruit juices is a ______________ chemical change.

2. Answer the following questions.

a. What type of change is photosynthesis?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

b. What do you understand by the physical properties of a substance?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

c. Explain why rusting of iron objects is faster in coastal areas than in deserts.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

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Worksheet 2

1. Why setting of curd is regarded as a chemical change?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

2. What is rust? What are the two important conditions for rusting?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

3. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

4. Classify the changes involved in the following processes as physical or chemical.

a. Dissolving sugar in water ________________

b. Burning coal ________________

c. Beating aluminium to make aluminium foil ________________

d. Formation of manure from leaves ________________

e. Respiration ________________

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PART I

A 1. (c) 2. (a) 3. (a) 4. (c)

5. (d) 6. (c) 7. (c) 8. (d)

B 1. chemical 2. undesirable

3. evaporation 4. desirable

5. faster 6. reversible

7. chemical

C 1. false 2. true 3. true 4. false

5. true 6. false 7. false 8. false

D 1. (d) reversible change

2. (b) chemical change

3. (a) non-periodic change

4. (c) periodic change

5. (e) physical change

E 1. endothermic change

2. desirable change

3. undesirable change

4. physical change

5. periodic change

PART II

A 1. Melting of ice cream

2. Cooking of food

3. Evaporation is essential for the water cycle that brings rain. It is a slow process in nature which brings humidity in the atmosphere. It helps in heat transfer. But, evaporation also makes the water level to dry up, especially in dry areas, where it is essential.

4. Cooking of food, curdling of milk

5. Respiration

B 1. Greater surface area, less humidity/moisture, high speed of wind, high temperature and lower air pressure are the factors that favour evaporation.

2. When a substance changes from the solid state to the vapour state without melting, then it is called sublimation.

3. Changes that occur in nature repeatedly after regular intervals of time and whose occurrence can be predicted are called periodic changes. For example, phases of the moon and heartbeat.

Changes that can be easily reversed by removing the cause of the change are called reversible changes. Evaporation is a reversible natural change as water can be obtained by cooling the vapours.

4. A chemical change that usually converts iron into iron oxide, called rust, when it is exposed to oxygen and moisture in air is called rusting.

5. Changes that cannot be easily reversed are called irreversible changes. Germination of seed is an irreversible change because a seed has developed into a complete plant.

6. Digestion is the process by which food is broken down into simpler chemical compounds that can be absorbed and used as nutrients or eliminated by the body. Digestion is a chemical change which is desirable, irreversible and endothermic.

C 1. There are some factors that favour evaporation.

Surface area: A larger evaporating surface increases the rate of evaporation.

Humidity/moisture: Drier air evaporates more water than humid air. For example, wet clothes do not dry easily in the rainy season, as air is already humid.

Motion of air: Wind accelerates evaporation.

Temperature: Warmer the evaporating surface, higher the rate of evaporation.

Air pressure: Lower pressure on the open surface of the liquid results in the higher rate of evaporation.

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2. (a) Desirable changes: Changes that are useful to mankind are said to be desirable changes. For example, evaporation is the process in which a substance in the liquid state changes into the gaseous state at any temperature. Water can be obtained by cooling the vapours. It is a desirable change, as it maintains the water cycle.

(b) Undesirable change: Change that bring about destruction are said to be undesirable changes.

For example, rotting of fruits, souring of milk and fermentation of fruit juices. In fermentation of juice, the juice becomes sour and unfit for drinking and it leads to spoilage.

(c) Exothermic changes: Changes that release energy in the form of heat are called exothermic changes. Burning is a chemical change in which a substance combines with oxygen at a high temperature, releasing light and heat. So, the burning of a candle is an exothermic change.

(d) Enothermic changes: Changes that absorb energy in the form of heat are called endothermic changes. Energy is absorbed as the products are formed. For example, digestion of food in which food is broken down into simple chemical compounds that can be absorbed and used as nutrients or eliminated by the body.

3. (a) Digestion of food is a chemical change by which food is broken down into simpler chemical compounds. It is a useful change as the simpler compounds formed can be absorbed and used as nutrients or eliminated by the body.

(b) Cooking of food is a chemical change because cooked vegetables cannot be changed into raw vegetables or spices. It is a useful change as cooked food is easy to digest and we have to use less energy to digest what we eat.

(c) Rusting of iron is a chemical change that usually converts iron into iron oxide, called rust, when it is exposed to oxygen and moisture in air is called rusting. It is harmful as it damages the stuff.

(d) Curdling of milk is a chemical change in which a milk changes into a soft, semi-solid mass is called curdling. It may be undesirable if it leads to the spoilage of milk and desirable if there is a need for curd.

(e) Fermentation is the chemical breakdown of a substance by bacteria, yeasts or other microorganisms, involving the release of carbon dioxide and heat. It may be useful or harmful. The fermentation of bread dough, idli and dosa batter is useful while the fermentation of fruit juices is harmful as the juice becomes sour and unfi t for drinking and it leads to spoilage.

4. Take two test tubes, two stirrers, water, washing soda and glucose.

Method: Put a teaspoon of washing soda in the first test tube. Add 10 mL water in it and stir till it dissolves. Feel the lower part of the test tube. It is hot. Now add a teaspoon of glucose in the second test tube. Add 10 mL water in it and stir. Touch the test tube. It is cold.

Observations: The test tube with water and washing soda will appear warm. The test tube with glucose and water will appear cold.

Conclusion: The dissolution of washing soda in water is a chemical change where heat is evolved (exothermic). Therefore, the test tube becomes hot. The dissolution of glucose in water is an endothermic chemical change where heat is absorbed. Therefore, the test tube becomes cold.

5. Curdling of milk is a chemical change as curd is chemically different from milk. This means a new substance is formed and curd cannot be changed into milk, so it is an irreversible change.

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6. The melting of solid wax to form liquid wax is a physical change. On burning, wax changes into vapour and other by products along with heat and light. This is a chemical change. So a burning candle shows both physical and chemical change.

D 1. By blowing air around, the fan makes it easier for the air to evaporate sweat from our skin, thus eliminating body heat. The more evaporation, the cooler we feel.

2. Evaporation is an endothermic process as water absorbs heat energy to change into vapour and hence evaporation results in cooling.

3. Though digestion takes place inside our body, it is a natural change. The fi rst stage of digestion is ingestion, in which food is taken into the mouth and then broken down into smaller pieces by the chewing action of the teeth. Usually, the sensations of sight, taste, and smell associated with food set in motion a series of neural responses that induce the formation of saliva by the salivary glands in the mouth which begins the process of breaking complex carbohydrates into simple sugars. Thus, the process of digestion continues.

4. Boiled eggs undergo chemical changes while cooking. The boiled egg cannot be changed back to original raw egg, so, it’s a chemical change.

E 1. A change that releases energy in the form of heat is called an exothermic change. The reactants have more energy than the products. Energy is released as the products are formed. As a result,the surroundings get heated.

A change that absorbs energy in the form of heat is called an endothermic change. The products have more energy than the reactants. Energy is absorbed as the products are formed. As a result, the surroundings get cooled.

2. Melting, or fusion, is a physical process that is reversible and results in the state transition of wax from a solid to a liquid. It

occurs at a specific temperature called the melting point.

The burning of a candle is a chemical change that is irreversible and exothermic. On burning, wax changes into vapour and other byproducts along with heat and light.

3. Curdling is the process by which a liquid changes into a soft, semi-solid mass. Curd is chemically di�erent from milk, so a new substance is formed.

Fermentation is the chemical breakdown of a substance by bacteria, yeasts or other microorganisms, typically involving the release of carbon dioxide (in the form of bubbles) and heat.

4. Changes that can be easily reversed by removing the cause of the change are called reversible changes. Melting of ice, stretching of rubber band and drying of clothes are the examples of reversible change.

Changes that cannot be easily reversed are called irreversible changes. Burning of matchstick, germination of seed are irreversible changes.

5. Changes that occur repeatedly after regular intervals of time and whose occurrence can be predicted are called periodic changes. Movement of pendulum and the hands of clocks, phases of the moon and heartbeat are periodic changes.

Changes that do not repeat themselves at regular intervals of time are called non periodic changes. Natural phenomenon like earthquakes, cyclones and volcanic eruptions are non periodic changes.

LEARNING ZONE


1. (a) Melting of wax

(b) Burning of candle (wax vapour)

(c) Burning is a chemical change in which a substance combines with oxygen at a high temperature, releasing light and heat.

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(d) The wax acts as a fuel when we light the candle and is basically carbon. The carbon combines with oxygen to form another chemical substance, carbon dioxide and water.

2. (a) B is the fermented dough.

(b) It is due to a process known as fermentation which is the chemical breakdown of a substance by bacteria, yeasts or other microorganisms, typically involving the release of carbon dioxide (in the form of bubbles) and heat.

(c) It is a chemical change which is brought about by enzymes.

(d) Fermentation involves the action of desirable microorganisms, or their enzymes, on food ingredients to make biochemical changes, which cause signifi cant modifi cation to the food.

3. Reversible: melting of ice, making pots

Irreversible: rusting of iron, curdling of milk, burning of wood

Non-periodic changes: All the changes that are given in the question are non-periodic.

Desirable changes: making of pots,

Undesirable changes: rusting of iron,

Physical changes: melting of ice, motion of bird, making pots

Chemical changes: burning of wood, curdling of milk, rusting of iron

Think and answer

1. In an earthen pot, water gets evaporated quickly through the pores. Cooling is caused by evaporation. Some heat energy is utilised during the process of evaporation. Since this energy is taken from the water itself, it leads to a lowering of temperature in the remaining amount of water. Mud pots by their very nature include a number of very minute pores through which water can slowly ooze out. In effect, these pores increase the surface area of water and consequently increase evaporation.

2. During the summer, humidity is very high. High humidity means the air is holding a lot of water. But there is a limit to the amount of water air can hold. Normally, the body cools itself by opening pores on the skin and releasing water and salts. As the water evaporates, it transfers the body’s heat to the air.The rate of sweating is directly related to our body temperature. The body overheats more easily in hot weather. When we get hot, our sweat glands are triggered and they release a watery substance that is the foundation of sweat. When sweat is released onto the skin and is exposed to the air, it will evaporate and ultimately help to cool down the body and lower our body temperatures.

3. When an apple is cut open, an enzyme called polyphenol oxidase is released from the cells of the apple and reacts with the oxygen in the air. This reaction causes the fruit to turn brown, similar to rust forming on metal.

Answers to Worksheets

WORKSHEET 1

1. a. chemical

b. physical; chemical

c. chemical; physical

d. rust

e. desirable, irreversible and endothermic

f. undesirable

2. a Photosynthesis is a chemical change which is desirable.

b. Properties like size, shape, colour and state of a substance are known as physical properties.

c. In coastal areas, there is more moisture in air due to the presence of water. In deserts, there is shortage of water and

2121

hence, air is almost dry. For rusting, both air (oxygen) and moisture are essential. So, rusting is faster in coastal areas than in deserts.

WORKSHEET 2

1. Curd is formed by adding some sour substance to milk and keeping it undisturbed for some hours. Some useful bacteria help in the setting of curd. Curd cannot be converted into milk. It is a different substance than milk. Hence, formation of curd is a chemical change.

2. Rust is an iron oxide. It consists of hydrated iron (III) oxide Fe2O3.nH2O.

Presence of both water and oxygen are required to form rust on iron.

3. Burning of wood produces ash, smoke which are new substances having entirely different chemical properties. It is reversible as wood cannot be got back from the ash and smoke. Hence, burning of wood is a chemical change. But when wood is cutting into small pieces, no new substance is produced. It undergoes a change in the physical properties such as its shape and size. Hence, it is a physical change.

4. a. Physical change

b. Chemical change

c. Physical change

d. Chemical change

e. Chemical change

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Learning Objectives

Students will be able to: learn about the elements, compounds and their representation

understand the formation of ions and different types of bonds as ionic and covalent bonds

know about the mixtures and how these are different from compounds

learn about different types of mixtures

understand separation techniques

learn about chromatography

3 Elements, Compounds and Mixtures

Warm-upStart the chapter by interacting with students about the things around them such as a steel glass, lemon juice, a piece of chalk, a wooden stick, an iron nail, smoke from a car. Now, ask them which of these are composed of a single substance and which are the combination of two or more. Let them recall the following:

Matter exists as:

mixture: homogeneous and heterogeneous

pure substance: elements and compounds

Now, ask the students to complete the exercise given in the Warm-up section.

Concept Explanation

ElementsExplain to the students that elements are pure substances made up of only one kind of atom. Some elements, such as helium, argon exist as single atoms and some cannot exist as single atoms and combine with one

or more atoms and form molecules of that element like nitrogen, sulphur.

Inside the AtomMake students aware of the fact that an atom itself is made up of even smaller particles called subatomic particles: protons, neutrons and electrons. Protons are positively charged particles, neutrons have no electrical charge and electrons are negatively charged particles. Protons and neutrons form the central core of an atom which is commonly called the nucleus and electrons surround the nucleus.

Representation of ElementsExplain to the students about the representation of elements with the help of symbols. Make them aware of atomicity and valency. The number of atoms present in a molecule of an element is called atomicity and valency is the combining capacity of an atom of an element. It is determined by the number of electrons an atom gains or loses.

2323

Explain by taking the example of an oxygen atom. It can combine with two hydrogen atoms to form one molecule of water. Hence, its valency is 2.


CompoundsExplain to the students that compounds are formed when the atoms of two or more different elements combine in a fixed ratio. A bond formed between them is ionic or covalent. Ionic bond is formed when electron transfer takes place which results in the formation of cation and anion. The positive and negative ions are bonded together to form a compound. For example, sodium chloride is formed by the combination of sodium and chloride ions.

Make students aware of the fact that sometimes, compounds are formed by the sharing of electrons between elements which are called covalent compounds and the chemical bond formed by the sharing of electrons between the atoms is called a covalent bond. Explain by taking the example of water. A hydrogen atom has one electron to share while oxygen has two electrons. So, two hydrogen atoms combine with one oxygen atom. Therefore, the valency of oxygen is 2 and that of hydrogen is 1.

Polyatomic Ions Explain to them about polyatomic ions or radicals. For example, CO3

–2 (carbonate ion), SO4

–2 (sulphate ion), PO4– (phosphate ion).

Representation of CompoundsMake students practice writing the molecular formulae.


Mixtures and its typesTell students about the differences between mixture and compounds and different types of mixtures.

Solid–solid mixture: Example, rice and sand; dal and rice; soil, salt and flour

Solid–liquid mixture: Example, sugar syrup, salt solution, starch solution

Liquid–liquid mixture: Example, lemon juice, alcohol and water; oil and water; milk and water

Gas–solid mixture: Example, smoke and dust particles in air

Gas–liquid mixture: Example: aerated drinks, oxygen dissolved in water

Gas–gas mixture: Example: air, acid vapour in air, water vapour in air

Explain to the students that depending on the appearance, mixtures can be divided into two types: homogeneous mixtures and heterogeneous mixtures.

In a homogeneous mixture, the components cannot be differentiated by colour, shape or size as it is a mixture having a non-uniform composition. Explain to them about solution and alloys. A solution is a homogeneous mixture of two or more substances. The components can be separated by evaporation or crystallization. An alloy is a homogeneous mixture of two or more metals or a metal and a non-metal.

Suspension and emulsion are examples of heterogeneous mixture. Suspension is a mixture containing solid particles that can be separated by filtration. For example, sand in water or chalk in water, whereas two immiscible liquids form an emulsion. In most cases, one of the components is water. A mixture of oil and water is an emulsion.


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Separation of MixturesExplain to students that there is a need for separating mixtures as a part of purification. Handpicking, sieving and winnowing are the methods that we use in our daily life and certain separation techniques are used in laboratories.

Separating Solid-Solid Mixtures: Sublimation is one of the techniques used to separate a solid-solid mixture which is a change from the solid to the vapour state without melting. A mixture of ammonium chloride and salt is separated by this method.

Separating Solid–Liquid Mixtures: Evaporation and distillation are the techniques which are used to separate the components of a solid-liquid mixture. Evaporation is the process in which a liquid changes into a gas at a temperature below its boiling point. When a salt solution is heated, water evaporates, leaving the salt behind.

Distillation is the method of separation of a liquid from a soluble component by fi rst converting the liquid into its vapour and then condensing the vapour. Distilled water used in laboratories and batteries is obtained using this method.

Separating Liquid–Liquid mixtures: The components of a liquid–liquid mixture can be separated by using a separating funnel or by fractional distillation.

Separating Funnel – When a liquid mixture is poured into the funnel and allowed to stand, it separates into different layers. A mixture containing oil and water is separated using this method.

Fractional Distillation - It is used to separate mixtures comprising miscible liquids like alcohol and water and is based on the difference in the boiling points of the liquids. Crude petroleum oil is refined using this principle.

Combination of methods for separationExplain to the students that a mixture which has more than two components can be separated into individual components by a combination of the methods of separation. Explain by taking example of the mixture of iron filings, ammonium chloride and sand.

Chromatography • Let students know about chromatography

which is one of the latest techniques to separate the coloured components of a mixture when all the components are very similar in their properties. The process of separating the different dissolved constituents of a mixture by their adsorption on an appropriate adsorbent is called chromatography. The common adsorbents used are filter paper, silica gel, etc. The common solvents used are water, ethyl alcohol, acetic acid, etc.

• The principle involved in chromatography is that it separates the components of a mixture on the basis of the differences between two phases, one of which is stationary while the other is mobile. The simplest type of chromatography is ‘paper chromatography’ in which the filter paper acts as a stationary phase and the solvent is the mobile phase.

• Paper chromatography is commonly used in the medical field, industrial field, forensic labs and commercial field.

ReinforceTo reinforce, ask students to read the Summary and Glossary sections and do the Exercises and Learning Zone section from the textbook. Help students develop skills (research and thinking) given in the Skills for Life section in the chapter.


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Worksheet 1

1. Classify each of the following as elements(E), compounds(C) or mixtured(M).

a. Diamond b. Sugar d. Milk e. Air

f. Sulphuric Acid (H2SO4) g. Gasoline h. Krypton (K)

i. Water j. Alcohol (CH3COOH) k. Pail of Garbage

l. Ammonia (NH3) m. Salt (NaCl) n.Wood o. Bronze

p. Dry Ice (CO2) q. Baking Soda (NaHCO3) r. Iron (Fe)

s. Popcorn t. Gold (Au)

2. On which principle is the process of chromatography based?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

3. How will you separate salt from a mixture of sand, salt and grain?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________


a. ____________ are positively changed particles present in an atom.

b. The symbol of gold is ____________.

c. ____________ bond is formed when electron transfer takes place.

d. Compounds that are formed by the sharing of electrons between elements are called ____________ compounds.

e. Steel is an alloy of ____________ and ____________.

f. ____________ are also called solid solutions.

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Worksheet 2

1. a. What is the name of the given apparatus?

b. Which type of mixture is separated by this method?

c. Give one example of the mixture separated by the given method.

________________________________________________________

________________________________________________________

________________________________________________________

2. Give one word for the following

a. techniques to separate the coloured components of a ________________ mixture when all the components are very similar in their properties.

b. a change from the solid to the vapour state without melting. ________________

c. a homogeneous mixture of two or more substances. ________________

d. compounds formed by the sharing of electrons ________________ between elements.

e. an atom or a group of atoms that gains electron(s). ________________

3. Write the molecular formula of compound Calcium Chloride.

4. What is distillation? When do we use distillation?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

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PART I

A. 1. (c) 2. (b) 3. (b) 4. (a)

5. (d) 6. (c) 7. (a) 8. (a)

B. 1. heterogeneous 2. chromatography

3. symbols 4. covalent

5. positively 6. anion

7. fractional distillation

C. 1. (c), 2. (e), 3. (a), 4. (b), 5. (d)

D. 1. false 2. true 3. true 4. true

5. true 6. true 7. false

PART IIA. 1. Atom itself is made up of even smaller

particles called fundamental particles or subatomic particles. They are protons, neutrons and electrons.

2. When iron fi lings and sulphur are heated together, a compound called iron sulphide is formed.

3. Fe2 (SO4)3– Fe—Fe3+ ; valency=+3

SO4—SO42– ; valency= –2

4. In the industrial field, chromatography is used for testing and identifying chemicals, separating dye stu�s, testing the purity of drugs, etc.

5. Example of Solid–liquid mixture: sugar syrup, salt solution.

6. Example of suspension is sand in water or chalk in water.

B. 1. Valency is the combining capacity of an atom of an element. It is determined by the number of electrons an atom gains or loses.

2. An ionic bond is formed by the transfer of one or more electrons from a metal to a non-metal. It can also be referred to as the attractive force that binds oppositely charged ions together.

For example, sodium chloride is formed by the combination of sodium

and chloride ions. On losing one electron, sodium forms a positively charged ion or sodium cation.

Na → Na+ (sodium ion) + e–

On the other hand, the chlorine atom gains one electron to form a negatively charged ion or chloride anion.

Cl + e– → Cl– (chloride ion) These oppositely charged ions attract each

other and combine together to form the sodium chloride (NaCl) compound.

3. An alloy is a homogeneous mixture of two or more metals or a metal and a non-metal. Alloys are obtained by melting the metals together. Brass is an alloy of copper and zinc. Steel is an alloy of iron and carbon.

4. Two or more atoms combine to form a molecule. The number of atoms present in a molecule of an element is called atomicity.

Inert gases are monoatomic and their atomicity is one.

5. Nitrogen (N) 3, 5 Iron (Fe) 2, 3 6. Chromatography is commonly used in

the medical field for separating amino acids, separating components of urine, identifying hormones, analysing blood samples and the like.

It is used in the commercial field for testing meat samples, testing the purity of canned food items, identifying adulterants in food, etc.

C. 1. Mixtures are to be separated as a part of purification. Sometimes, there is a need to obtain a useful component from a mixture of materials and sometimes, harmful materials like adulterants needs to be removed from food materials. In industry too, there is a need to obtain chemically pure materials.

The different types of mixtures are-

Solid-Solid Mixtures, Solid–Liquid Mixtures, Liquid–Liquid Mixtures

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2. A mixture of ammonium chloride, salt and sand is taken in a china dish. It is covered with a funnel. The tail of the funnel is plugged with cotton wool. The china dish is heated on a tripod stand.

Ammonium chloride vaporises and gets deposited near the tail end of the funnel. Salt and sand is left in the china dish. Now, we are left with the mixture of salt and sand. Add some water and make a solution. The solution is filtered to obtain sand (residue) and a salt solution. Sand is insoluble in water and can be easily removed by filtration. Salt can be obtained from the salt solution by evaporation.

3. Alcohol can be separated from water by the process of fractional distillation.

It is based on the difference in the boiling points of the liquids.

The distillation flask is fitted with a fractionating column, which is connected to the condenser and a thermometer is inserted into the fractionating column. The temperature across the column varies; it is higher at the bottom and lower at the top. The vapours of the component with a higher boiling point condense at the bottom of the column. The liquid with the lower boiling point vapourises first and rise up. The vapours of the other liquid coming along in small quantities will be condensed and sent back into the flask by the fractionating column. The vapours of the liquid with the low boiling point rise and are condensed in the condenser. This liquid is then collected in a container. After all the first component is removed, the temperature starts rising till the boiling point of the second liquid is reached. Its vapours are also condensed and collected in a di�erent container.

4. The two types of chemical bonds are-

Ionic and covalent bond.

An ionic bond is formed by the transfer of one or more electrons from a metal to a non-metal. It can also be referred to as the attractive force that binds oppositely charged ions together. When an atom

loses one electron, it imparts a positive charge to the resulting elements. This is called a positive ion or cation. When an atom gains one electron, it becomes a negative ion called anion. The positive and negative ions are bonded together to form a compound.

For example, sodium chloride is formed by the combination of sodium and chloride ions. On losing one electron, sodium forms a positively charged ion or sodium cation.

Na → Na+ (sodium ion) + e–

On the other hand, the chlorine atom gains one electron to form a negatively charged ion or chloride anion. Cl + e– Cl– (chloride ion)

These oppositely charged ions attract each other and combine together to form the sodium chloride (NaCl) compound.

Compounds that are formed by the sharing of electrons between elements are called covalent compounds and the chemical bond formed by the sharing of electrons between the atoms is called a covalent bond.

For example- the formation of water.

Formation of water: H* + *O* +*H → H**O**H (H2O)

A hydrogen atom has one electron to share while oxygen has two electrons.

So, two hydrogen atoms combine with one oxygen atom. Therefore, the valency of oxygen is 2 and that of hydrogen is 1.

5. (a) Calcium Chloride-

(a) Write the symbols side by side

Ca Cl

(b) Write the valencies below each

+2 –1

(c) Interchange the valencies and write as subscripts

Ca1 Cl2

(d) Ratio of valencies in the lowest terms

1 : 2

(e) Chemical formula of calcium chloride

CaCl2

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(b) Carbon dioxide-


C O


+4 –2


C2 O4


1 : 2

(e) Chemical formula of carbon dioxide

CO2

(c) Magnesium carbonate-


Mg (CO3)


+2 –2


Mg2 (CO3)2


1 : 1

(e) Chemical formula of Magnesium carbonate

MgCO3

6. If the liquid component is required in a pure form, the solution is subjected to distillation. The method of the separation of a liquid from a soluble component by first converting the liquid into its vapour and then condensing the vapour is called distillation. The distillation apparatus comprises a distilling flask which is a round-bottomed flask with a side tube. The side tube is connected to a Liebig condenser and a receiver. A thermometer is inserted into the flask. On heating, the liquid component vaporises and vapours get cooled as they pass through the condenser. The condensed liquid collects in the receiver. The solid component remains in the flask. In evaporation, the liquid component of a mixture turns into vapour and is lost to the atmosphere. In distillation, the liquid component can be recovered.

7. The process in which a liquid changes into a gas at a temperature below its boiling point is called evaporation.

Evaporation is the technique which is used to separate the components of a solid-liquid mixture. In this method, the liquid part of a mixture evaporates and the solid part is left behind when the mixture is heated. For e.g. When a salt solution is heated, water evaporates, leaving the salt behind.

D. 1. A magnet attracts all the iron filings mixed with sulpur from the mixture, leaving only the sulphur behind. The compound iron sulphide formed by heating iron and sulphur cannot be separated using a magnet.

2. The size of the starch particles is very small, they appear homogeneous when placed in solution. But appear heterogenous under a microscope and do not settle down like suspensions. Hence, starch particles cannot be separated from starch solution by fi ltration.

3. Alloying a metal with another gives it more durability and strength. Alloys are also called solid solutions.

4. Nitrogen, sulphur, oxygen, carbon, etc. exist as molecules. These elements that cannot exist as single atoms combine with one or more atoms and form molecules of that element.

5. Fractional distillation is based on the di�erence in the boiling points of the liquids. The distillation flask is fitted with a fractionating column, which is connected to the condenser and a thermometer is inserted into the fractionating column. The temperature across the fractionating column varies; it is higher at the bottom and lower at the top. The vapours of the component with a higher boiling point condense at the bottom of the column. The vapours of the other liquid coming along in small quantities will be condensed and sent back into the fl ask by the fractionating column.

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E . 1. Two immiscible liquids form an emulsion. In most cases, one of the components is water. A mixture of oil and water is an emulsion. When one of the components is thicker and present in excess, the emulsion is a semi-solid or a paste-like mass. Face creams, shaving creams and butter are examples of emulsion.

A suspension is a heterogeneous mixture containing solid particles that are suffciently large for sedimentation. The components can be separated by filtration. An example of a suspension would be sand in water or chalk in water. The suspensions can be separated into their components.

2. In homogeneous mixture, the components cannot be differentiated by colour, shape or size. They are evenly distributed. The mixture appears as one single entity. For example, in a salt solution, we cannot see salt particles. The solution appears just as a liquid

A heterogeneous mixture is a mixture having a non-uniform composition, with at least two phases that remain separate from each other, with clearly identifiable properties. Separate components are visible. Ice cubes in cola form a heterogeneous mixture.

3. Atom is the basic unit of matter and is the smallest component of an element.

Atoms are made up of three particles: protons, neutrons and electrons and atom as a whole is electrically neutral.

Ions are charged particles formed by the loss or gain of electrons by an atom or group of atoms. An atom that loses electron(s) forms a positive ion and is called a cation. An atom or a group of atoms that gains electron(s) forms a negative ion and is called an anion.

4. If the liquid component is required in a pure form, the solution is subjected to distillation. This method includes separation of a liquid from a soluble component by first converting the liquid into its vapour and then condensing the vapour.

Fractional Distillation is used to separate mixtures comprising miscible liquids like alcohol and water. It is based on the difference in the boiling points of the liquids.

5. A solution is a homogeneous mixture of two or more substances. The components can be separated by evaporation or crystallisation. The substance that is dissolved is called a solute and the substance in which it is dissolved is called a solvent.

An alloy is a homogeneous mixture of two or more metals or a metal and a non-metal. Alloys are obtained by melting the metals together.

LEARNING ZONE


1. (a) 1. Thermometer 2. Fractionating column 3. Water out 4. Condenser

5. Roundbottomed flask 6. Water in

7. Bunsen burner 8. Distillate

(b) It is based on the di�erence in the boiling points of the liquids.

(c) Miscible liquids like alcohol and water are separated by this method.

2. (a) Paper Chromatography

(b) Paper, Ink spot, Water

(c) In the industrial field, it is used for testing and identifying chemicals, separating dye stuffs, testing the purity of drugs, etc.

It is used in forensic labs for testing blood samples, cloth samples and other body fluids collected from crime sites.

It is used in the commercial field for testing meat samples, testing the purity of canned food items, identifying adulterants in food.

3. (a) (Suspension)

(i) It is a heterogeneous mixture.

(ii) The components can be separated by filtration.

(iii) Chalk in water

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(b) Emulsion

(i) It is a heterogeneous mixture.

(ii) Components can be separated by separating funnel

(iii) A mixture of oil and water is an emulsion.

Think and answer

1. Copper Sulphate solution has copper sulphate crystals dissolved in water. Oil and copper sulphate solution when poured into the funnel and allowed to stand, separates into different

layers . Oil is separated by this method. Now copper sulphate solution is subjected to evaporation and pure copper sulphate crystals are obtained.

2. Alloying a metal with another gives it more durability and strength. Alloy of aluminium is used for making aeroplane parts because it is a high strength aluminium alloy. It has excellent fatigue resistance. It also has very good corrosion resistance and fi nishing ability.

3. Na is sodium atom which is electrically neutral, whereas Na+ is a sodium ion. A sodium atom loses one electron to form sodium ion.

Answers to WorksheetsWORKSHEET 1

1. a. E b. C c. M d. M

e. C f. M g. E h. C

i. C j. M k. C l. C

m. M n. M o. C p. C

q. E r. M s. E

2. Chromatography is based on the principle of selective adsorption (surface absorption). It separates the components of a mixture on the basis of the differences between two phases, one of which is stationary while the other is mobile.

3. A mixture of salt, sand and grain can be separated into individual components by a combination of the methods of separation. Grain is separated from the mixture by sieving. Now, mixture of salt and sand is left. Add some water and make a solution. The solution is fi ltered to obtain sand (residue) and a salt solution. Sand is insoluble in water and can be easily removed by filtration. Salt can be obtained from the salt solution by evaporation.

4. (a) Protons (b) Au (c) Ionic

(d) convalent (e) iron, carbon

(f) Allows

WORKSHEET 2

1. a. Separating Funnel

b. Liquid—liquid mixture

c. A mixture containing oil and water is separated using this method.

2. a. Chromatography b. Sublimation

c. Solution d. Covalent

e. Anion

3. Calcium Chloride


Ca Cl


+2 –1


Ca1 Cl2


1 : 2

(e) Chemical formula of carbon dioxide

CaCl2

4. The method of separation of a liquid from a soluble component by fi rst converting the liquid into its vapour and then condensing the vapour is called distillation.

If the liquid component is required in a pure form, the solution is subjected to distillation.

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Learning Objectives

Students will be able to: learn about the atoms, molecules and radicals

understand the significance of valency to write the formulae of compounds

know the arrangement of elements into groups and periods in the Periodic table

learn about the modern periodic law

differentiate between atomic number and mass number

4 Atomic Structure

Warm-upStart the chapter by revising the elements and their symbols, the concept of atomicity and valency.

Now, ask students to complete the exercise given in the Warm-up section.

Concept Explanation

Atoms, Molecules and RadicalsExplain to the students that atom which is the smallest particle of any matter and responsible for its properties, cannot exist independently without combining with other atoms or molecules.

Let the students know the difference between atoms and molecules. Atoms of the same or different elements combine to form molecules. A molecule is the smallest particle capable of independent existence and responsible for the properties of an element or a compound of which it is a part. Explain to them by giving example of water.

Explain to the students about polyatomic ions or radicals and the charge they carry.


Valency Students should be clear with the concept of valency which is the combining capacity of an element or in other words, is the charge carried by an ion. Valencies help in the derivation of the chemical formulae of compounds.

Explain to them by giving examples of compounds like zinc bromide, magnesium chloride, etc.

Periodic TableExplain to the students that the ‘Modern Periodic Law’ states that the properties of elements are a periodic function of their atomic number and the elements are arranged in increasing order of their atomic numbers. This is in contrast to ‘Mendeleev’s Periodic Law’ which was based on atomic mass. There is a systematic arrangement of elements into groups (vertical columns) and periods (horizontal rows). Let students

3333

analyse the periodic table and understand how different elements are placed and the also observe colour scheme of the table. Make them understand that different colours show different groups like metals, non-metals, metalloids, inert gases, etc.

Inside an AtomExplain to the students that the number of protons in an atom of an element forms the atomic number and the total number of protons and neutrons present in the nucleus of an atom of an element forms the mass number. The electrons are present around the nucleus and the electrons that are present

in the outermost shell are called valence electrons and the outermost shell is known as the valence shell.

ReinforceTo reinforce, the students to read the Summary and Glossary sections and do the Exercises and Learning Zone sections from the textbook. Help students develop skills (research and thinking) given in the Skills for Life section in the chapter.


3434

Worksheet 1

1. Carbon has a mass number 12. What are the number of protons and neutrons?

________________________________________________________________________

________________________________________________________________________

2. State modern periodic law.

________________________________________________________________________

________________________________________________________________________

3. In the periodic table, what is a –

a. Period :

________________________________________________________________________

________________________________________________________________________

b. Group :

________________________________________________________________________

________________________________________________________________________


a. ______________ forms a cloud around the nucleus.

b. Electrically charged atoms are ______________.

c. An atom’s nucleus contains ______________.

d. The outermost shell of an atom is called ______________.

e. ______________ and ______________ do not form molecules.

f. A ______________ ion is formed when an atom adds an electron.

g. The systematic arrangement of elements into groups and periods is called the ______________.

h. ______________ showed that the atomic number of an element is a more fundamental properties than its mass.

i. Elements were classifi ed into ______________ based on the number of valence electrons.

3535

Worksheet 2

1. Give two differences between atoms and radicals,

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

2. Lithium, sodium and potassium have one electron in their valence shell. To which group these elements belong? What is the nature of their hydroxides?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

3. Give one word for the following.

a. Indivisible particles, of which elements are made _________________

b. Number of protons in an atom _________________

c. The charge carried by an ion _________________

d. These are present around the nucleus _________________

e. The ion formed when an atom gains an electron _________________

4. Defi ne ions. How are positive ion and negative ion formed?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

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3636


PART I

A. 1. (b) 2. (c) 3. (b) 4. (b)

5. (b)

B 1. 11 2. atoms 3. 2nd; 3rd

4. ammonium 5. increasing

C 1. True 2. False 3. True

4. False 5. False

D 1. Nitrogen

2. Ion

3. Polyatomic ions or radicals

4. Valence electrons

5. Mass number

PART IIA 1. There are 8 elements in the second period

of the periodic table.

2. Valency is the combining capacity of an element or in other words it is the charge carried by an ion.

3. Elements in the modern periodic table are arranged in 7 periods and 18 groups.

4. Elements are arranged in increasing order of their atomic numbers. Elements with similar atomic structure (and, hence, similar chemical properties) appear in vertical columns.

5. Neon and Argon

6. Valency of nitrogen in ammonia is 3.

B 1. The color represents what type of element it is. It shows the nature of element in the table like noble gas, alkali metal, alkaline earth, metalloid etc.

2. The valency of nitrate ion in nitric acid is 1.

3. The electrons that are present in the outermost shell or valence shell are called valence electrons.

4. Elements are arranged in increasing order of their atomic numbers.

Horizontal rows in the periodic table are called periods and vertical columns in the table are called groups.

5. Inert gases are stable due to having the maximum number of valence electrons their outer shell can hold. Therefore, they do not react with other elements.

C 1. The systematic arrangement of elements into groups and periods is called the periodic table. In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass as given by Mendeleev’s Periodic Law. The modern periodic law is stated as: ‘Properties of elements are a periodic function of their atomic number’. The atomic number is the number of protons in the nucleus; it is also equal to the number of electrons in the atom. The main features of the modern periodic table are as follows.

(a) Elements are arranged in increasing order of their atomic numbers.

(b) Horizontal rows in the periodic table are called periods and vertical columns in the table are called groups.

(c) Elements in the modern periodic table are arranged in 7 periods and 18 groups.

(d) Elements are so arranged that elements with similar atomic structure (and, hence, similar chemical properties) appear in vertical columns.

(e) Elements were classifi ed into groups based on the number of valence electrons. It means that all the elements within a certain group have the same number of valence electrons. So, the elements belonging to the same group share similar properties. For example, lithium, sodium and potassium belong to group 1,so these elements have one electron in their valence shell.

3737

2. Valency is the combining capacity of an element or it is the charge carried by an ion. Valencies help significantly in the derivation of the chemical formulae of compounds. For example, in the case of zinc bromide, when two elements such as zinc (valency: +2) and bromine (valency: –1) combine to form zinc bromide, the chemical formula of the compound (ZnBr2) is simply the interchange of their valencies to obtain the ratio of their combining atoms or groups.

3. 2nd group includes-Be, Mg, Ca, Sr, Ba, Ra 18th group includes-He, Ne, Ar, Kr, Xe, Rn 4. Refer Table 4.1, page 55D. 1. Inert gases have a completely fi lled valence

shell and do not combine with other elements to form compounds. Hence, the valency of inert gases is zero.

2. Some atoms form ions due to their electron confi guration, to achieve a more stable state i.e. to complete their octet.

3. This is so because in an atom number of positively charged particles (protons) is equal to number of negatively charged particles (electrons), so net charge is 0 i.e. an atom is neutral.

4. A molecule as compared to the atoms from which it is formed is more stable because it possesses energy lower than the energy of the uncombined atoms. This difference in energy is due to the fact that when atoms combine to form molecule, the attractive forces are created which result in release of energy. A molecule is thus, the smallest particle capable of independent existence and responsible for the properties of an element or a compound of which it is a part.

E 1. A positive ion formed when an atom loses an electron is called a cation.

e.g. Na → Na+ + e– A negative ion formed when an atom adds

an electron is called an anion. Cl + e– → Cl–

2. Horizontal rows in the periodic table are called periods.There are 7 periods in the

periodic table.

Vertical columns in the table are called groups. There are 18 groups in the periodic table.

3. The number of protons in an atom of an element is called atomic number (denoted by Z).

The total number of protons and neutrons present in the nucleus of an atom of an element is called mass number (denoted by A).

4. Molecules can exist independently whereas radical cannot exist independently Molecules are stable whereas radical is highly reactive. Molecules are electrically neutral whereas radicals carry either positive or negative charge.

LEARNING ZONE

Picture based questions1. (a) 7 periods

(b) 18 groups

(c) 18th group includes– He, Ne, Ar, Kr, Xe, Rn.

Their valency is zero.

These elements are called inert gases or noble gases.

(d) These elements have one electron in their valence shell, so have valency of 1.

(e) Hg

(f) Nitrogen

(g) An element of the fi rst period with two valence electrons is- He.

2. A—hydrogen molecule

B—water molecule

C—hydrogen atom

D—H3O+--radical

Think and answer

1. Since chlorine has 7 electrons in its valence shell, it would take much less energy to acquire an electron and reach the stable 8 electrons in a shell confi guration rather than losing its 7 electrons. Hence the valency of Chlorine is one.

3838

2. The elements in Group 1 of the periodic table are called the alkali metals. They form ionic compounds when they react with non-metals. Their ions have a single positive charge. For example, sodium forms sodium ions, Na+.

The elements in Group 17 of the periodic table are called the halogens. They form ionic compounds when they react with metals. Their ions have a single negative charge. For

example, chlorine forms chloride ions, Cl–.

3. Yes, it is possible and these are called Isotopes. Isotopes are atoms of an element with the normal number of protons and electrons, but different numbers of neutrons. Isotopes have the same atomic number, but different mass numbers.

For eg. 1H1,

1H2, 1H

3

Answers to Assessment 1A. 1. (a) 2. (b) 3. (b) 4. (d) 5. (c)B. Defi ne the following words. 1. Compound - Compounds are formed

when the atoms of two or more different elements combine in a fi xed ratio. The properties of a compound are different from that of the combining atoms. For example, magnesium chloride is formed by the combination of magnesium and chlorine.

2. Alloy - An alloy is a homogeneous mixture of two or more metals or a metal and a non-metal. Alloys are obtained by melting the metals together. Brass is an alloy of copper and zinc.

3. Dissolving - The process of mixing a solid, liquid or gas uniformly with another substance to form a solution is called dissolving. This process does not take place when any solid or liquid mix but takes place only when the right solid mixes with the right liquid. When they do dissolve, the solid part is called the solute and the liquid in which it dissolves is called the solvent.

4. Covalent bond - Compounds that are formed by the sharing of electrons between elements are called covalent compounds and the chemical bond formed by the sharing of electrons between the atoms is called a covalent bond. For example, the formation of water, carbon dioxide and ammonia.

5. Chromatography - The process of separating the different dissolved constituents of a mixture by their adsorption on an appropriate adsorbent is called chromatography.

C 1. Mass number Total number of protons and neutrons

2. Polyatomic ion Nitrate

2. Valence electron Electrons present in the outer shell

4. Group 2 Magnesium

5. Period 3 Sodium

D 1. Solid

2. Endothermic change

3. Phosphorous

4. Desirable changes

5. Valency

E 1. Suspensions and emulsions –

A suspension is a heterogeneous mixture in which the solute particles do not dissolve but get suspended throughout the bulk of the medium. The components can be separated by filtration. Particles of the suspension are visible to the naked eye. A suspension is formed when particles are left floating around freely in a solvent. An example of a suspension would be sand in water or chalk in water.

3939

Emulsion - Two immiscible liquids form an emulsion. In most cases, one of the components is water. A mixture of oil and water is an emulsion. When one of the components is thicker and present in excess, the emulsion is a semi-solid or a paste-like mass. Face creams, shaving creams and butter are examples of emulsion. They are all mixtures of fat with water.

2. Elements and compounds-

Elements are pure substances made of only one kind of atom. They cannot be broken down further by chemical reactions.

Compounds are formed when the atoms of two or more different elements combine in a fixed ratio. The properties of a compound are different from that of the combining atoms.

3. Exothermic change and endothermic change-

A change that releases energy in the form of heat is called an exothermic change. The reactants have more energy than the products. Energy is released as the products are formed. The container in which an exothermic change is taking place will be warm.

A change that absorbs energy in the form of heat is called an endothermic change. The products have more energy than the reactants. Energy is absorbed as the products are formed. The container in which an endothermic change is taking place will be cold.

4. Interparticle force of attraction and interparticle space-

Interparticle space is the vacant space between the molecules whereas interparticle force is the attractive force between the molecules.

In case of solids, interparticle space is minimum or absent and interparticle force is maximum.

F 1. False 2. False 3. False

4. True 5. True

G. 1. Matter is anything that occupies space and has mass. The quantity or the amount of matter that a body has is called its mass.

2. Matter is composed of either a pure substance or a combination of two or more pure substances. A pure substance has a fixed chemical composition throughout and also has well-defined physical properties.

3. The process by which a liquid turns into a vapour when it is heated to its boiling point is called boiling. Boiling is a physical change. It is a reversible, fast and endothermic change.

4. The process that converts iron into iron oxide, called rust, when exposed to oxygen and moisture in air is called rusting. It is a chemical change.

5. An element is the simplest form of matter in that it cannot be further broken down using any chemical means. Elements are made up of smaller particles called atoms.

6. Valency is the combining capacity of an atom of an element. It is determined by the number of electrons an atom gains or loses. For example, an oxygen atom can combine with two hydrogen atoms to form one molecule of water. Hence, its valency is 2.

7. Horizontal rows in the periodic table are called periods and vertical columns in the table are called groups.

Elements in the modern periodic table are arranged in 7 periods and 18 groups.

H. 1. Matter exists in three forms: solid, liquid and gas.

Solids- A solid has a definite shape and a fixed volume. A solid can be rigid or fl exible, hard or soft, compressible or non-compressible and light or heavy. Solids retain their shape unless they are acted upon by an external agent such as pressure, force and heat. A wooden block, a rubber band, a piece of sponge, a chalk piece, a piece of candle, rice flour, sand, etc. are some examples of solids.

4040

Liquid -A liquid has a fixed volume, but it does not have a fixed shape. Liquids take the shape of the container they are poured into. A liquid flows. Water, juice, milk, oil, tea, etc. are some examples of liquids.

Gas - A gas has neither a definite shape nor a defi nite volume. Gases cannot be confined to an open container. Most of the gases cannot be seen with the naked eye. Air, carbon dioxide, oxygen, nitrogen, steam, hydrogen, etc. are some examples of gases Liquids and gases are also called fluids as they can flow.

2. Uses of Paper Chromatography

- Chromatography is commonly used in the medical fi eld for separating amino acids, separating components of urine, identifying hormones, analysing blood samples and the like.

- In the industrial field, it is used for testing and identifying chemicals, separating dye stu�s, testing the purity of drugs, etc.

- It is used in forensic labs for testing blood samples, cloth samples and other body fluids collected from crime sites.

- It is used in the commercial field for testing meat samples, testing the purity of canned food items, identifying adulterants in food, etc.

3. The main features of the modern periodic table are-

- Elements are arranged in increasing order of their atomic numbers.

- Horizontal rows in the periodic table are called periods and vertical columns in the table are called groups.

- Elements in the modern periodic table

are arranged in 7 periods and 18 groups.

- Elements are so arranged that elements with similar atomic structure (and, hence, similar chemical properties) appear in vertical columns.

4. Just as ions are formed when neutral atoms gain or lose electrons, a polyatomic ion is formed when a neutral molecule gains or loses electrons. Thus, sometimes, a group of atoms behave like a single unit carrying a charge. Hence, they are highly reactive.

5. If the liquid component is required in a pure form, the solution is subjected to distillation. The method of the separation of a liquid from a soluble component by first converting the liquid into its vapour and then condensing the vapour is called distillation.

The distillation apparatus comprises a distilling flask which is a round-bottomed flask with a side tube. The side tube is connected to a Liebig condenser and a receiver. A thermometer is inserted into the flask. On heating, the liquid component vaporises and vapours get cooled as they pass through fl ask.

I. 1. This apparatus is the laboratory set-up of distillation.

2. A. Distilling flask

B. Condenser

C. Distillate

3. Distillation is a technique used to separate the components of a solid-liquid mixture. The method of the separation of a liquid from a soluble component by first converting the liquid into its vapour and then condensing the vapour is called distillation.

4141


WORKSHEET 1

1. Mass number = No. of protons + No. of neutrons 6, 6

2. Modern Periodic Law states that -

Properties of elements are a periodic function of their atomic number.

3. The seven horizontal rows of the periodic table are called periods. The vertical columns starting from top to bottom are called groups. There are 18 groups in the periodic table.

4. a. Electrons

b. Ions

c. Protons and neutrons

d. Valence shell

e. Metals and inert gases.

f. negative

g. periodic table

h. Henry Moseley

i. groups

WORKSHEET 2

1. Atom Radicals1. Electrically

neutral1. Carry either

positive or negative charge

2. May or may not react

2. Highly reactive

2. Li, Na, K belong to Group 1. The hydroxides of these group elements are basic in nature and soluble in water.

3. a. Atom

b. Atomic number

c. Valency

d. Electrons

e. Anion

4. Ions are charged particles formed by the gain or loss of electrons by an atom. A positive ion is formed when an atom loses an electron. A negative ion is formed when an atom adds an electron.

4242

Total marks: 50Time: 3 hours

Model Test Paper 1

Section AA. Fill in the blanks. [5]

1. A pure substance can either be an ______________ or a _________________.

2. In solids, the intermolecular attraction holds the molecules ________________.

3. A physical change is __________________ and easy to reverse.

4. A larger evaporating surface __________________ the rate of evaporation.

5. Fermentation involves the release of _________________ and ____________________.

B. Multiple choice questions. [5]

1. Rusting is a chemical change which is

a. reversible b. irreversible

c. exothermic d. irreversible and endothermic

2. A change in which heat is evolved is called a

a. Exothermic change b. Endothermic change

c. Periodic change d. Desirable change

3. Negatively charged particles in an atom are known as

a. Electrons b. Neutrons c. Protons d. Ions

4. Metals and inert gases are monoatomic as their atomicity is

a. one b. varies between 1 and 2 c. four d. two

5. Chemical formula of ammonium is

a. NH3 b. NH3+ c. NH4

+ d. NH4+2

C. State True or False. [5]

1. Compounds are formed when the atoms of two or more different elements combine in a fi xed ratio.

2. Elements with valency 1 are called monovalent.

3. Covalent compounds are formed when cations combine with anions.

4. The constituents of a mixture can be separated by physical methods.

5. The number of protons and neutrons present in the nucleus of an atom of an element is called atomic number.

D. Name the following. [4]

1. Vertical columns in the Periodic table

2. Subatomic particles without any charge present in the nucleus

3. A mixture of metal with another metal or non-metal

4. The process in which food breaks down to release energy, water and carbon dioxide

4343

Section BA. Defi ne the following. [3]

1. Separating funnel

2. Melting

3. Exothermic change

B. Short answer questions. [12]

1. Why boiling, evaporation, and condensation are physical changes?

2. How is salt extracted from seawater? What is this process known as?

3. Differentiate between Ionic and covalent bonds.

4. State the difference between valency and atomicity.

C. Long answer questions. [12]

1. Explain the process of fractional distillation.

2. What happens when sugar is melted? Is it a physical change or a chemical change?

3. Cott on plug

Inverted funnel

Solidifi ed ammonium chloride

Vapour of ammonium chloride

China dishAmmonium chloride salt

Burner

a. Defi ne the above process.

b. What does this process show?

c. What happens to ammonium chloride?

d. What is left in the china dish?

e. List the steps to write the chemical formula of magnesium oxide and calcium chloride.

4444

Learning Objectives

Students will be able to: understand what are chemical reactions and know about different types of chemical

reactions

differentiate between combination and decomposition reaction

learn about various changes that occur during a chemical reaction

understand chemical equations and their representation

5 Language of Chemistry

Warm-upStart the chapter by asking students the difference between physical and chemical changes. Let them explore the concept of chemical change by giving various examples like rusting (oxidation), cooking, baking. Now let students answer questions like – why is there a formation of gas which can be seen by bubbling, a change of colour, precipitate formation, etc.

Now, let students complete the exercise given in the Warm-up section.

Concept Explanation

Chemical ReactionsExplain to the students that in a chemical reaction a substance changes into a new one that has a different chemical identity. Such reactions take place in the presence of heat, light, electricity or catalysts. When this happens, old bonds are broken and new bonds are formed.

Types of Chemical ReactionsExplain to the students, the different types

of reactions - combination reactions, decomposition reactions, single displacement reactions, double displacement reactions, neutralisation reactions and combustion.

Make them understand the difference between combination and decomposition reactions. In a combination reaction, two or more reactants combine to form a more complex product. In a decomposition reaction, a compound breaks up into simpler compounds or elements.

Characteristics of Occurrence of Chemical ReactionsExplain to the students that several visible changes occur during a chemical reaction like change of colour, evolution of gas, evolution of smell, formation of precipitate, occurrence of fi re and evolution of heat or cooling effect.

To show the evolution of gas - When baking soda reacts with acetic acid, carbon dioxide (gas) and water are formed. CO2 gas is evolved with strong effervescence and turns lime water milky.

4545

Activity - activities can be shown to children regarding the evolution of gas and heat during chemical reactions.


Chemical EquationsElements or compounds that take part in a chemical reaction are called reactants and those that are formed as a result of a chemical reaction are called products. For example, iron and sulphur react to form iron sulphide. In this reaction, iron and sulphur are reactants and iron sulphide is product.

Explain to the students the difference between the word equation and chemical equation. The representation of a chemical reaction using symbols and formulae is called a chemical equation whereas when a chemical reaction is represented using only the names of the chemicals involved, it is called a word equation.

Let students understand that a chemical equation is based on the law of conservation

of mass that is the number of atoms of each element is the same on both sides of the chemical equations.

Explain to the students different types of chemical reactions showing their word equation and chemical equation. For example,

Reaction between hydrogen and chlorine (combination reaction):

Word equation: Hydrogen + Chlorine

→ Hydrogen chloride

Chemical equation: H2 + Cl2 → 2HCl

ReinforceTo reinforce, ask students to read the Summary and Glossary sections and do the Exercises and Learning Zone section from the textbook. Help students develop skills (research and thinking) given in the Skills for Life section in the chapter.


4646

Worksheet 1

1. Write the chemical equations for the following word equations.

a. Magnesium + Oxygen →heat Magnesium oxide

b. Iron sulphide + hydrochloric acid → hydrogen sulphide + Iron Chloride

c. Hydrogen + Chlorine → hydrogen chloride

d. Iron (II) + Sulphur → Iron (II) sulphide

2. What is a balanced equation? Give one example.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

3. Why should a magnesium ribbon be cleaned before burning in air?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

4. Write an experiment that shows the evolution of gas during a chemical reaction.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

5. Write two chemical reactions in which the change of colour occurs.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

4747

Worksheet 2

1. How does a chemical reaction take place?

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

2. What are the different experimental conditions required for a chemical reaction to take place.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

3. Fill in the blanks:

a. A chemical reaction can be represented by a _____________ equation and a

_____________ equation.

b. A reaction in which two or more reactions combine to form a new product is

called _____________ reaction.

c. _____________ are written on the left-hand side and _____________ are written

on the right-hand side of the chemical equation.

d. _____________ of iron changes the colour of iron metal.

e. _____________ gas turns lime water milky.

4. Write an experiment that shows the evolution of gas (smell) during a chemical reaction.

________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

4848


PART I

A. 1. (b) 2. (a) 3. (a) 4. (a)

5. (a) 6. (b)

B 1. Decomposition 2. HCl

3. Carbonic Acid 4. Heat

5. Reactants

C 1. False 2. True 3. False

4. True 5. False

D 1. 2Ca + O2 → 2CaO

2. CO2 + H2O → H2CO3

3. CaO + H2O → Ca(OH)2

4. 2C + O → 2CO

5. C + O2 → CO2

PART IIA 1. The substances that take part in a chemical

reaction are called reactants.

2. The substances that are formed as a result of chemical rection are called products.

3. When a drop of iodine solution is added to starch, the starch turns black, thus colour change occurs.

4. Calcium carbonate on heating decomposes into calcium oxide and carbon dioxide.

5. When a chemical reaction is represented using only the names of the chemicals involved, then it is called a word equation. For example,

Magnesium + oxygen →heat

Magnesium oxide

A chemical equation is the shorthand form of representing a chemical reaction using symbols and formulae.

2Mg + O2 →heat 2MgO

B 1. The representation of a chemical reaction using symbols and formulae is called a chemical equation. The substances that take part in a reaction are called reactants

and the substances that are formed as a result are called products.

Reaction between sodium and chlorine (combination reaction) →

2Na + Cl2 → 2NaCl

2. In a decomposition reaction, a compound breaks up into simpler compounds or elements in the presence of heat. For example, on heating, calcium carbonate decomposes into calcium oxide and carbon dioxide.

3. There are several visible changes that occur during a chemical reaction.

Change of colour

Evolution of gas

Evolution of smell

Formation of precipitate

Occurrence of fire

Evolution of heat or cooling effect.

4. (a) Combination reaction

(b) Yes, it is a balanced equation.

(c) CO2 and H2O are reactants.

(d) Carbonic acid is the product.

5. (a) H2 + Cl2 → 2HCl

(b) C +O2 → CO2

(c) 2H2O → 2H2 + O2

C. 1. (a) When baking soda reacts with acetic acid, carbon dioxide gas is evolved with strong effervescence which turns the lime water milky.

Sodium bicarbonate + acetic acid →carbon dioxide +water

(b) If silver nitrate solution is poured into a solution of sodium chloride, a chemical reaction occurs, forming a white precipitate of silver chloride

Silver nitrate + Sodium chloride →Silver chloride +Sodium nitrate

(c) When sodium metal is kept outside,

4949

they burn spontaneously. Sodium will rapidly oxidize when exposed to a limited amount of oxygen, such as is found in air, to form sodium oxide.

Sodium + oxygen → sodium oxide (Na2O)

In addition, if the air is moist and contains a lot of water, sodium can react violently with the water content of air to produce sodium hydroxide and hydrogen gas + a lot of heat which usually results in fi re...

Sodium+water → sodium hydroxide + hydrogen + Heat (fi re)

2. Direct Combination Reaction (synthesis reaction) -- In a combination reaction, two or more reactants combine to form a more complex product. The combination of iron and sulphur to form iron (II) sulphide is an example of a combination reaction.

8Fe + S8 → 8FeS

Decomposition Reaction. In a decomposition reaction, a compound breaks up into simpler compounds or elements in the presence of heat. For example, on heating, calcium carbonate decomposes into calcium oxide and carbon dioxide .

CaCO3 →heat CaO + CO2

3. (a) Iron(II) + Sulphur → Iron (II) sulphide

Chemical equation:

Fe + S →heat FeS

(b) Hydrogen + Chlorine →Hydrogen chloride

Chemical equation:

H2 + Cl2 →Sunlight 2HCl

D. 1. Hydrogen and oxygen gases mix at room temperature with no chemical reaction. This is because the speed of the molecules does not provide enough kinetic energy to activate the reaction during collisions between the reactants.

Thus, hydrogen does not react with oxygen at room temperature, a source of energy is needed to ignite the mixture.

2. When Iron nail (Ferrum) is dipped in

copper sulphate (CuSO4), there takes reaction between them and copper sulphate change its colour from blue to light green that is blue colour fades. This shows iron is more reactive than copper, it can replace copper from CuSO4. CuSO4 is blue in colour and FeSO4 is light green in colour.

E. 1. Combination and decomposition reactions: In a combination reaction, two or more reactants combine to form a more complex product. The combination of iron and sulphur to form iron (II) sulphide is an example of a combination reaction.

Whereas in a decomposition reaction, a compound breaks up into simpler compounds or elements in the presence of heat. For example, on heating, calcium carbonate decomposes into calcium oxide and carbon dioxide .

2. Reactants and products: In a chemical reaction, the substances that take part are called reactants and the substances that are formed as a result of the reaction are called products. Reactants are written on the left-hand side with a plus sign in between and products are written on the right-hand side with a plus sign between them.

LEARNING ZONE

Picture based questions1. (a) Iron and sulphur are the reactants.

(b) Iron sulphide is the product.

(c) Combination reaction

(d) Fe + S → FeS

2. (a) The nail looks brown due to the formation of a reddish brown layer on the iron nail.

(b) This process is called rusting which is a combination reaction.

Think and answer

1. The gases hydrogen and chlorine do not react with each other even if kept together for a long time. But in the presence of sunlight they readily combine as the sunlight provides energy to break the bonds between atoms of hydrogen and chlorine and then they react to

5050

form hydrogen chloride.

2. Phosphorus is a very reactive metal, when exposed to air it starts reacting vigrously, and also catches fi re, therefore it is kept in water.

3. Apples contain an enzyme (called polyphenol oxidase or tyrosinase) that reacts with oxygen

and iron-containing phenols that are also found in the apple. The oxidation reduction basically forms a sort of rust on the surface of the fruit. We generally see the browning when the fruit is cut or bruised because these actions damage the cells in the fruit, allowing oxygen in the air to react with the enzyme.


WORKSHEET 1

1. a. 2Mg + O2 →heat 2MgO

b. FeS + 2HCl → FeCl2 + H2S

c. H2 + Cl2 → 2HCl

d. Fe + S → FeS

2. A chemical equation in which the number of atoms of each element is the same on both sides is called balanced equation. For e.g. H2 + Cl2 → 2HCl

3. Magnesium ribbon is cleaned before burning to remove the coating of impurities such as oxide so that reaction takes place properly.

4. Refer to textbook page 67.

5. a. Rusting of iron changes the colour of iron metal.

b. When a drop of iodine solution is added to starch. The starch turns black

WORKSHEET 2

1. A chemical reaction is the change of a substance into a new one that has a different chemical identity. Elements and compounds react under favourable conditions to form new compounds. In this process, old bonds are broken and new bonds are found.

2. Different experimental reactions require different experimental conditions such as light, heat, electricity, catalyst or pressure.

3. a. Chemical; word b. combination

c. Reactants; products d. Rusting

e. Carbon dioxide

4. Refer to textbook page 68.

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Learning Objectives

Students will be able to: understand what are chemical reactions and different types of chemical reactions

differentiate between combination and decomposition reactions

learn about various changes that occur during a chemical reaction

understand chemical equations and their representation

6 Metals and Non-metals

Warm-upStart the lesson by asking the students whether they are familiar with a number of materials like iron, aluminium, copper, carbon, gold, silver, etc. Let them recall various properties like lustre, malleability, ductility, etc. and then differentiate various elements into metals and non-metals. Now let students complete the exercise given in the Warm-up section.

Concept Explanation

Metals and their PropertiesExplain to the students that elements are classifi ed into metals, non-metals and metalloids.

Make them understand that metals and non-metals have opposing properties. Metals are hard and shiny, electropositive, have high melting point and boiling point, good conductor of heat and electricity, and are etc.

Non-metals and their PropertiesExplain to the students that non-metals are soft and dull, electronegative, non-sonorous,

non-ductile, bad conductor of heat and electricity, etc.

Make students understand the difference between metals and non-metals.


Uses of Metals and Non-metalsExplain to the students the various uses of metals like in jewellery, thermometers, galvanization, wires, dentistry, etc.

Explain to the students about various uses of non-metals giving example of carbon. It mainly occurs as coal, graphite, diamond, charcoal, coke.

Let them also know about oxygen, hydrogen, sulphur, phosphorus, inert gases and their various uses. Inert gases are used in the manufacture of insecticides, freezing of food, weather observation balloons, electric bulbs, etc.


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CorrosionExplain to the students that the oxidation of some metals causes corrosion. It can be prevented by applying oil or paint, electroplating or galvanisation.

Metalloids and their UsesExplain to the students about metalloids. They resemble metals in their physical properties and non-metals in their chemical behaviour. Make them understand their uses in semiconductor, paints, electric bulbs, making bullets, etc.

ReinforceTo reinforce, ask students to read the Summary and Glossary sections and do the Exercises and Learning Zone sections from the textbook. Help students develop skills (research and thinking) given in the Skills for Life section in the chapter.


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Worksheet 1


a. ______________ is used in non-stick cooking utensils.

b. Tinning involves coating with ______________.

c. ______________ is the only radioactive inert gas used for cancer treatment.

d. ______________ is used to preserve food, tissues and human organs.

e. ______________ is necessary for the combustion of fuels.

2. How is oxygen useful in the medical fi eld?

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3. Give 2 uses of graphite.

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4. Name two metals that exist in free state in nature. What are they known as?

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5. What are inert gases. Name them.

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Worksheet 2

1. Explain why metals are electropositive and non-metals are electronegative.

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2. Name non-metals that show variable valencies.

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3. `Copper objects get coated with a green substance with the passage of time’.

a. What is this green substance known as?

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b. Why is this formed?

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4. List 4 methods to prevent the corrosion of metals.

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5. Give 2 differences between non-metals and metalloids.

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PART 1

A 1. (a) 2. (b) 3. (a) 4. (a)

5. (d) 6. (a) 7. (b)

B 1. Sodium 2. Tungsten

3. Diamond 4. Graphite

5. Zinc 6. Duralumin

7. Bleaching 8. Hydrogen

C 1. False 2. False 3. True

4. False 5. True 6. False

7. True 8. True

D 1. Tungsten 2. Graphite

3. Antimony 4. Graphite

5. Stainless steel 6. Silver/gold

PART 2A 1. Elements are classified into metals, non-

metals and metalloids.

2. Neon is used in advertising signboards.

3. Diamond and Graphite

4. A homogeneous mixture of two or more metals or a metal and a non-metal is known as an alloy. For example,

5. Charcoal

6. Rusting occurs in the presence of oxygen and moisture in the atmosphere.

7. Chlorine gas is used as a disinfectant for drinking water.

8. Diamond

B 1. Metals are hard and shiny materials, that generally do not occur independently. Metals are electropositive and highly reactive. For example, iron, tin, copper.

2. Rusting of metals can be prevented by:

• Applying oil or grease

• Applying paint

• Galvanization (coating with zinc)

• Tinning (coating with tin)

• By electroplating (coating with a less reactive metal like chromium)

• By alloying (making alloys)

3. The process of the slow eating away of a metal due to the attack of atmospheric gases and moisture on its surface is called corrosion. It is also known as slow oxidation. Rusting is an example of corrosion.

4. The different forms of carbon are:

Coal. It is used as a fuel in homes and industries and in thermal power plants for generating energy.

Graphite. It is used as an electrode material in electrolytic cells because it is a good conductor of electricity.

Diamond. Pure diamond is used as a precious gem in jewellery.

Charcoal. It is used in water filters to absorb foul-smelling gases. It is also used as a fuel.

Coke. It is used in metallurgy to extract metals from their compounds.

5. The alloys of aluminium are magnelium and duralumin. They are used in aeroplane parts.

6. Lead is known as systemic toxicants which is known to induce multiple organ damage, even at lower levels of exposure.

7. The properties of metalloids are as follows.

• They have metallic lustre.

• They are brittle.

• Metalloids such as silicon and germanium are semiconductors under certain conditions.

• They form alloys with metals.

• They are solids at room temperature.

• They are similar to non-metals in their chemical behaviour.

C. 1. Almost all metals are solid at room

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temperature. Mercury, gallium, francium, caesium and rubidium are the only metals that occur in liquid state at room temperature.

• Metal are generally hard and strong. Sodium, potassium and magnesium are soft metals.

• Metals have lustre

• They are malleable. Tungsten is an exception as it is a non-malleable metal.

• They are ductile. Lead and zinc are non-ductile metals

• They are sonorous.

• Metals generally have high melting and high boiling points. Sodium and potassium are exceptions.

• They are good conductors of heat and electricity. Lead is an exception

• Most of the metals combine with oxygen to form oxides.

2. (a) Lead is an exception as metals are good conductors of electricity whereas it is a bad conductor of heat and electricity.

(b) Non-metals are bad conductors of heat and electricity whereas graphite is a good conductor of electricity.

(c) Non-metals are electronegative and form anions during chemical reactions. Hydrogen forms a positive ion just like metals.

(d) Almost all metals are solid at room temperature. Mercury occur in liquid state at room temperature.

(e) Non-metals exist as solids or gases. Bromine, a non-metal, exists as a liquid at room temperature.

3. Elements that behave like both metals and non-metals are called metalloids. They are solids at room temperature. Boron, silicon, germanium, antimony, selenium and tellurium are the known metalloids.

Some of the uses of metalloids are as follows-

Silicon is used in solar energy devices,

computer chips and transistor diodes. It is also used to make non-stick cooking utensils.

Germanium is used to form alloys in fluorescent lamps. Antimony is used in fire crackers, and in making bullets. Arsenic is poisonous and is used in making rat poison. Tungston is used in electric bulbs.

4. (a) Copper. Copper is used to make wires that carry electric current. Copper metal is used as electrodes and wires are used in heaters.

(b) Aluminium. Aluminium is a silvery white, light metal used for making window and door frames, kitchen utensils and aluminium sheets and aluminium foils for wrapping food items. The alloys of aluminium are used in aeroplane parts.

(c) Chlorine. Chlorine is used as a disinfectant for drinking water.

(d) Graphite. Graphite is a crystalline, black, lustrous substance. It is soft and soapy and turns paper black. Graphite is used the lead of pencils, as an electrode material in electrolytic cells because it is a good conductor of electricity, as a solid lubricant for maintaining machine parts.

(e) Silicon. Silicon is the most widely used metalloid. It occurs in free state in nature. It is a semiconductor and is used in solar energy devices, computer chips and transistor diodes. It forms polymers called silicones which find various uses such as making food wrappers, bullet proof materials, waterproof clothing and artificial implants for our body. It is also used to make non-stick cooking utensils.

(f) Arsenic. Arsenic is poisonous. It is used in making rat poison. It is a slow poison that causes the deformation or paralysis of body and, ultimately, leads to death if taken continuously.

(g) Mercury. Mercury expands consider-ably even for a very small increase in

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temperature. This property is used to make it the thermometric liquid. Mer-cury is used in barometers which are used to measure atmospheric pressure and in thermometers.

D. 1. Aluminium is highly reactive metal. When aluminium is exposed to air it reacts with oxygen and forms a white thin layer of aluminium oxide on its surface. This layer forms protective coating on aluminium and prevents the reaction of aluminium with oxygen. Aluminium metal with this protective layer is used for making cooking utensils.

2. Silver objects turn black due to the formation of a layer of silver sulfi de (Ag2S) on their surface. Traces of hydrogen sulfi de (H2S) present in air react with silver to form silver sulfi de, which gives the metal black appearance.

3. Gold has lustre and is malleable. Hence, used for making jewellery.

4. Copper objects get coated with a green substance with the passage of time which is called copper carbonate. This forms due to the reaction of copper with the carbon dioxide and moisture present in the atmosphere. Tinning i.e., coating with tin is done to prevent it from corrosion.

5. Charcoal has the ability to absorb gases on its surface. This surface absorption is called adsorption. Thus, it is used in water filters to absorb foul-smelling gases.

6. Mercury expands considerably even for a very small increase in temperature. This property is used to make it the thermometric liquid.

E. 1. Metals Non metalsAlmost all metals are solid at room temperature except mercury.

Non-metals exist as solids or gases. Bromine, a non-metal, exists as a liquid.

They are good conductors of heat and electricity.

They are bad conductors of heat and electricity.

They are malleable and ductile.

They are non-malleable and non-ductile.

Metals are electropositive and form cations.

Non-metals are e lec t ronega t ive and form anions.

2. Coke is obtained from coal. It is a pure form of amorphous carbon. It is used in metallurgy to extract metals from their compounds.

Charcoal has the ability to absorb gases on its surface. It is used in water filters to absorb foul-smelling gases. It is also used as a fuel. Animal charcoal is used as a bleaching agent in the sugar industry to decolourise sugar solutions.

3. Diamond is one of the crystalline forms of carbon. It shines brilliantly and is the hardest naturally occurring substance known. Pure diamond is used as a precious gem in jewellery and impure diamond is used for cutting glass, grinding hard substances and in drilling heads.

Graphite is a crystalline, black, lustrous substance. It is soft (in contrast to diamond) and soapy and turns paper black. Graphite is used in the lead of pencils and as electrode material in electrolytic cells because it is a good conductor of electricity.

4. The major difference between adsorption and absorption is that one is a surface process and the other a bulk process. Adsorption is a surface process, the accumulation of a gas or liquid on a liquid or solid. Absorption is a phenomenon involving the bulk properties of a solid, liquid or gas. It involves atoms or molecules crossing the surface and entering the volume of the material.

5. The property by virtue of which metals can be beaten into thin sheets is called malleability and the property of metals by virtue of which they can be drawn into wires without breaking is called ductility.

6. Iron is a chemical element that is found in abundance in the Earth’s crust. Steel is an

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alloy, made by mixing iron with carbon; hence it is a byproduct of iron. However, these are still different from each other as iron is also used to make a variety of other alloys.

LEARNING ZONE

Picture based questions1. (a) Iron, steel, aluminium, copper, gold.

Metal are generally hard and strong. They have lustre, that is, they shine. They are malleable. i.e can be beaten into thin sheets.

They are ductile i.e they can be drawn into wires without breaking is called ductility. Gold and silver are the two most ductile metals known. Metals are sonorous, the property by virtue of which metals produce a ringing sound when struck with a hard object. They are good conductors of heat and electricity. Metals like aluminium and copper are used to make utensils for cooking.

2. (a) The nail in test tube A rusts because the nail is in contact with oxygen and water.

(b) There is no change in the the nail in test tube B i.e., it will not rust, as boiled water has no oxygen and oil prevents the atmospheric oxygen from coming in contact with the nail. The nail in test tube C is in contact with air only. The water vapour of the air is absorbed by anhydrous calcium chloride. Hence, the nail in test tube C will not rust.

3. (a) Diamond, Helium, Argon

(b) Diamond is one of the crystalline forms

of carbon. It shines brilliantly and is the hardest naturally occurring substance known to us so pure diamond is used as a precious gem in jewellery.

Helium is the second-lightest element known to man. It is filled in weather observation balloons.

Argon is filled in electric bulbs so as to provide an inert atmosphere to the heating filament. It improves the quality and life of the bulb.

(c) Helium and argon

Think and answer

1. These metals are called noble metals. This is because these metals do not react (or react very slowly) with oxygen in air to form oxides. The noble metals are inert to this reaction and thus remain all shiny and new for years. They are most ductile and most malleable and have high lustre which adds to their value.

2. Iron is the most widely used metal. It is a bright shining metal when freshly prepared. Steel is an alloy of iron with carbon that is used to make tools, implements, machine parts and surgical instruments. Stainless steel is another alloy that does not rust.

3. Tungsten is a non-malleable metal. It has high electrical resistance and becomes incandescent when electricity passes through it, so it is used in electric bulbs.

4. Production of lead paint is banned because as lead paint ages, it can chip or crumble into dust. Exposure to lead paint dust or chips can cause serious health problems, especially to children.


WORKSHEET 1

1. a. Silicon

b. Tin

c. Radon

d. liquid nitrogen

e. oxygen

2. It is the life-supporting gas. In hospitals, oxygen cylinders are used for artificial respiration.

3. Graphite is used

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As electrode material in electrolytic cells because it is a good conductor of electricity.

As a solid lubricant for maintaining machine parts.

4. Metals such as gold and platinum exist in free state in nature and are called non-reactive metals.

5. Inert gases are chemically inert, non-metallic, gaseous elements. They are found only in traces in air. They are six in number: helium (He), neon (Ne), xenon (Xe), argon (Ar), krypton (Kr) and radon (Rn).

WORKSHEET 2

1. Metals are electropositive as they form cations by losing electrons. Non-metals are electronegative as they form anions by

gaining electrons.

2. Carbon, sulphur, phosphorus

3. a. The green substance is called copper carbonate.

b. This is formed due to the reaction of copper with the carbon dioxide and moisture present in the atmosphere.

4. Corrosion of metals can be prevented by:

Applying oil or grease

Applying paint

Galvanisation (coating with zinc)

Tinning (coating with tin)

5. Non-metals don’t have lustre, so they look dull. Metalloids have metallic lustre. Non-metals exist as solids or gases whereas metalloids exist as solids at room temperature.

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Learning Objectives

Students will be able to: understand the composition of air

learn about nitrogen – its preparation and uses

know the steps involved in nitrogen cycle

learn about oxygen – its preparation, properties and uses

understand oxygen cycle in nature

know the uses of carbon dioxide

understand greenhouse effect

learn about air pollution, its causes and methods to reduce air pollution

7 Air and Atmosphere

Warm-upStart the chapter by asking students questions like what is air? What is the composition of air? Why is air known as mixture of gases? Then explain to them the importance of various components of air. Now, ask the students to complete the exercise given in the Warm-up section.

Concept Explanation

Composition of AirExplain to the students that air is one of the most important resources which is necessary for all living organisms. It is colourless and transparent. Various components of air include nitrogen, oxygen, carbon dioxide and water vapour.

NitrogenExplain to the students that nitrogen forms 78% of composition of air. It is essential for

plants and animals.

Explain to the students that nitrogen is prepared in the lab by compression followed by expansion of air, which produces cooling and thus nitrogen is collected. Make them aware of the fact that it is also prepared in the laboratory by passing purifi ed air through heated copper metal.

Explain to the students various uses of nitrogen.

Explain to the students the importance of nitrogen cycle that how nitrogen is renewed in the atmosphere with the help of photosynthesis. Make them understand various steps of nitrogen cycle like nitrogen fi xation, assimilation of soluble nitrates, ammonifi cation, nitrifi cation and denitrifi cation.


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OxygenExplain to the students about the second-most abundant gas in the atmosphere – oxygen, which is found both in free and in combined state.

Explain to the students that oxygen is prepared in the laboratory from hydrogen peroxide, potassium chlorate, potassium permanganate and water. Let them understand the physical and chemical properties of oxygen.

Explain to the students the difference between rapid oxidation (combustion) and slow oxidation (respiration). Make them understand various uses of oxygen like in artifi cial respiration, to burn fuels in rockets, welding torches, in marine life, etc.


Carbon DioxideExplain to the students various uses of CO2 – in photosynthesis, in the medical fi eld, in the industries (for making baking soda and washing soda, urea, etc.), storing food grains. Make them aware of the importance of CO2 in soft drinks.

Explain to the students about the greenhouse effect and how CO2 behaves like a glass and allows heat radiation to enter the surface of the earth and does not allow the

radiation refl ected by the Earth’s surface to escape (Global Warming).


Air QualityExplain to the students how undesirable gases (air pollutants) present in the air have harmful effect on plants and animals.

Give them a list of various air pollutants and explain to them how acid rain (containing H2SO4, carbonic acid, HNO3) is causing damage to various monuments.

Explain to them how Government is trying various methods to reduce air pollution like use of CNG in Delhi. Also make them understand planting trees, preparing compost, using public transport and car pooling can reduce air pollution.

ReinforceTo reinforce, ask the students to read the Summary and Glossary sections and do the Exercises and Learning Zone sections from the textbook. Help students develop skills (research and thinking) given in the Skills for Life section in the chapter.


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Worksheet 1

1. Give one word for the following.

a. Relationship where plants and bacteria are mutually _________________ benefi tted

b. Undesirable gases and particles present in the air _________________

c. Oxygen mixed with 5% carbon dioxide _________________

d. Chemical name for baking soda _________________

e. Gas known as ‘gift from plants’ _________________

2. Explain oxygen cycle.

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3. Why is baking powder used for baking purposes.

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4. How is dry ice important to us?

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Worksheet 2

1. Give 2 examples of slow oxidation.

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2. What is oxy-acetylene fl ame? Mention its use.

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3. What is a catalyst? Name one catalyst.

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4. What is acid rain? Mention the harmful effects caused by acid rain.

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5. Why is carbon dioxide used in fi re extinguishers?

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PART 1

A 1. (b) 2. (d) 3. (d) 4. (c)

5. (a) 6. (c)

B. 1. Photosynthesis 2. Rhizobium

3. Greenhouse effect 4. Metal oxide

5. Nitrogen fi xation 6. Carbon dioxide

C 1. True 2. False 3. True 4. True

5. True 6. False 7. True

D 1. Ammonia

2. Manganese dioxide

3. Oxygen

4. Combustion

5. Carbon Dioxide

6. Hydrated ferric oxide

7. Photosynthesis

E 1. Hydrogen

2. Photosynthesis

3. Manganese dioxide

4. Volcanic eruption

5. Oxygen

PART 2A 1. Nitrogen

2. TNT (trinitrotoluene), nitroglycerine.

3. Combustion of petrol

4. The steps involved in the nitrogen cycle are nitrogen fixation, assimilation of soluble nitrates, ammonification, nitrification and denitrification.

5. Carbon dioxide

6. For artificial respiration in hospitals and ambulances.

B 1. When oxides of nitrogen and sulphur mix with rainwater, they form nitric acid and sulphuric acid, respectively. This rain that contains acids such as sulphuric acid, carbonic acid and nitric acid is called acid

rain. Acid rain is harmful to the crops and aquatic organisms, which are not used to such toxic water. Acid rain corrodes metal structures, bridges, railway lines, etc.

2. Plants that have nodules on their roots are called leguminous plants, for example, peas, grams, beans, etc. The nodules of these plants contain nitrogen fixing bacteria known as Rhizobium. These bacteria have the property of absorbing nitrogen directly from the air and converting them into nitrates.

3. Ammonia is synthesized by the Haber–Bosch process. Nitrogen and hydrogen in the ratio 1 : 3 by volume compressed to 200–300 atmospheric pressure is passed over finely divided iron at 300 °C. The ammonia gas formed is cooled to obtain liquid ammonia.

N2 + 3H2 300—450 c

200-300 atm pressure→ 2NH3

4. Oxy-acetylene fl ame is produced when oxygen is mixed with acetylene and produces a flame with very high temperature. It is used in welding torches.

5. Uses of Carbon Dioxide:

(a) In fire extinguishers as it does not support combustion.

(b) Dry ice which is the solid form of carbon dioxide acts as a good refrigerant and is used to keep food items, such as vegetables and meat, frozen.

(c) In photosynthesis- green plants use carbon dioxide to carry out photosynthesis.

(d) In soft drinks- Carbon dioxide is made to dissolve under pressure in soft drinks. Soda water is formed when carbon dioxide dissolves in water.

C. 1. Oxygen can be prepared in the laboratory from hydrogen peroxide. It is the most convenient method as it does not involve heating. Hydrogen peroxide decomposes

6565

into water and oxygen on adding manganese dioxide. Manganese dioxide acts as a catalyst. Manganese dioxide initiates and speeds up the decomposition of hydrogen peroxide to release oxygen.

2H2O2 → 2H2O + O2

Refer fi gure 7.4

2. Uses of Nitrogen:

(a) Nitrogen is used to make calcium cyanamide (CaCN2), which is a raw material for urea production and used as a fertilizer.

(b) It is used for making azo dyes in the textile industry.

(c) Many medicines, including antibiotics, contain nitro compounds.

(d) It is used in food preservation. While canning and packing food items, nitrogen is used to provide an inert atmosphere within the sealed packs.

3. (a) Sodium

Sodium + Oxygen → Sodium oxide

4Na + O2 → 2Na2O

When sodium is exposed in air with normal temperature it reacts with oxygen in the air and white solid gets coated on the surface of the sodium metal. It is of sodium oxide (Na2O). It generates a lot of heat energy and light.

(b) Phosphorus

Phosphorus + Oxygen →Phosphorus pentoxide

4P(s) + 5O2 (g) → 2P2O5(s)

(c) Sulphur:

Sulphur + Oxygen → Sulphur dioxide

Sulphur burns in air or oxygen on gentle heating with a pale blue fl ame. It produces colourless sulphur dioxide gas. Note: Sulphur dioxide can, of course, be converted further into sulphur trioxide in the presence of oxygen, but it needs the presence of a catalyst and fairly carefully controlled conditions.

(d) Copper

copper + oxygen → copper oxide

The most common reaction of copper and oxygen results in a product of copper(II) oxide, or cupric oxide. The reaction is:

2Cu+O2 → 2CuO

Copper(II) oxide is the most stable of the two stable oxides of copper (the other stable oxide being copper(I) oxide or cuprous oxide, Cu2O.

4. The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into various chemical forms as it circulates among the atmosphere, terrestrial, and marine ecosystems. Important processes in the nitrogen cycle include fi xation, ammonifi cation, nitrifi cation, and denitrifi cation.

Nitrogen Fixation- The process of the conversion of free atmospheric nitrogen into its compounds is called nitrogen fixation. Plants cannot absorb nitrogen directly as such. The conversion of nitrogen into its compounds takes place as follows. Plants that have nodules on their roots are called leguminous plants, for example, peas, grams, beans, etc. The nodules of these plants contain nitrogen fixing bacteria called rhizobium which absorbs nitrogen directly from the air and converts them into nitrates. Nitrogen is returned to the soil when plants and animals decay. This process is carried out by denitrifying bacteria, which convert the proteinaceous matter to free nitrogen.

Refer fi gure 7.3

5. The release of air pollutants into the air through unwanted human activities or by natural processes is called air pollution.

Natural causes- Wind carries dust and sand particles from one place to another. During forest fires, large amounts of smoke and carbon dioxide enter the atmosphere.

During volcanic eruptions, a lot of ash and sulphur dioxide spread in the air.

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Pollen grains from plants are carried by the wind. This causes allergies in some people.

Human activities-

– Thermal power stations, steam engines and cement and sugar industries give out a large amount of smoke and carbon dioxide which spread over a radius of about 20 km.

– Automobiles give out a large amount of smoke, carbon dioxide and even carbon monoxide, lead particles and oxides of nitrogen.

– Burning of garbage, especially plastics, gives out toxic substances into the air.

– Pesticides sprayed in the fields also cause air pollution.

6. Carbon dioxide is used up by plants for photosynthesis. Seawater absorbs carbon dioxide and stores it as carbonates. Shells and corals are nothing but calcium carbonate.

D 1. Baking powder is used for baking purposes because it produces carbon dioxide in the dough, causing it to rise.

2. Due to greenhouse effect, heat is trapped inside the glass house, leading to a rise in temperature. Carbon dioxide behaves like glass. It allows heat radiations to enter the surface of the Earth but does not allow the radiations reflected by the Earth’s surface to escape. This results in ‘Global warming’. Thus, the atmospheric temperature increases all over the globe.

3. Elements, including metals, on oxidation combine with the oxygen present in air, in fi xed ratios by mass, to form binary compounds called oxides. The combining ratio also depends on the valency of the element. Since a metal always combines with the oxygen taken from air to form an addition product i.e. the oxide, it experiences a defi nite increase in mass.

4. Farmers grow leguminous plants in the fi eld in between two harvests because leguminous plants have nitrogen fi xing

bacteria at their root nodules which take up atmospheric nitrogen and convert them to nitrous compounds. This enriches the soil with nitrogen and reduces the use of chemical fertilizers.

E 1. Rapid oxidation and slow oxidation: The type of oxidation in which oxygen reacts very quickly with a substance with the release of heat and light energy is called rapid oxidation. Rapid oxidation is also called combustion. Often, heat is required to initiate rapid oxidation. Slow oxidation occurs so slowly at ordinary temperature that it produces no light and no noticeable heat. The rusting of iron, respiration and the rotting of wood are good examples of slow oxidation.

2. Combustion and oxidation: Combustion is a burning process of a substance in the presence of oxygen. It occurs with fl ame and with or without smoke. On the other hand, it is a process of addition of oxygen, removal of hydrogen or electron. It occurs with or without fl ame or smoke.

3. Metal oxides and non-metal oxides: Metallic oxides are chemical compounds of metals with oxygen like Na2O, MgO, while on the other hand, non metal oxides are chemical compounds of non metals with oxygen like CO2, NO2.

Metallic oxides are basic in nature because they react with dilute acids to form salt and water .They also react with water to form metal hydroxides which are alkaline in nature. On the other hand, non metal oxides react with water to form acidic solutions. Therefore, these are acidic in nature.

4. Nitrifi cation and denitrifi cation: In Nitrification Nitrosomonas bacteria convert nitrogen into nitrite (NO2–) and then Nitrobacter converts nitrite to nitrate (NO3–). This process is called nitrification. Plants utilise nitrates as a nutrient and animals obtain nitrogen by eating plants or plant-eating animals .

Denitrifi cation is the reduction of NO3– (nitrates) to gaseous N2 (free nitrogen)

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by anaerobic bacteria. This process only occurs where there is little or no oxygen, such as deep in the soil near the water table. This is the reduction of nitrates to gaseous nitrogen. Denitrifying bacteria, such as pseudomonas, perform almost the reverse of nitorgen-fixing bacteria.

5. Greenhouse effect and global warming: Excessive carbon dioxide, methane and water vapour cause the greenhouse effect that contributes to global warming. The phenomenon in which the sun’s radiation reflected from the Earth’s surface is trapped by gases in the lower atmosphere is called the greenhouse effect. Glass has the property of letting in heat radiation from the sun but not allowing it to go out. Thus, heat is trapped inside the glass house, leading to a rise in temperature. Here, carbon dioxide behaves like glass. It allows heat radiations to enter the surface of the Earth but does not allow the radiations reflected by the Earth’s surface to escape. This results in ‘Global warming’.

LEARNING ZONE

Picture based questions1. (a) (i) Tap funnel

(ii) Delivery tube

(iii) Conical flask

(iv) Glass cylinder

(v) Trough containing water

(b) Hydrogen peroxide – H2O2

Manganese dioxide –MnO2

(c) Oxygen gas does not dissolve easily in water, so collected over water.

(d) It is neutral to litmus.

2. (a) Metal cutting using an oxy-acetylene flame.

(b) For welding torches.

(c) When Oxygen is mixed with acetylene it produces a flame with very high temperature.

(d) People working with oxyacetylene fl ame

must wear safety glasses to protect the eyes from sparks or other debris.

3. (a) Nitrogen fixation: Fixed by lightning, nitrogen fixing bacteria and blue green algae.

(b) Assimilation: Absorption of nitrates by plants, and plants are consumed by animals

(c) Ammonification: Bacteria convert dead plant and animal waste into ammonium compounds.

(d) Nitrification: Nitrifying bacteria convert ammonium compounds into nitrates.

(e) Denitrification: Bacteria convert nitrates into free nitrogen.

Think and answer

1. Nitrogen is an inactive gas, still it is the major component of air, due to the following reasons:

– Nitrogen is volatile in most of its forms.

– It is unreactive with materials that make up the solid earth.

– It is very stable in the presence of solar radiation.

2. When an aerated drink bottle is opened, the gas gushes out along with the content of the bottle, with great force because carbon dioxide is made to dissolve under pressure in such drinks

3. (a) Green plants use carbon dioxide to carry out photosynthesis. If it is reduced in the air, the CO2 and O2 level will get disturbed.

(b) All living beings use atmospheric oxygen to breathe. Oxygen is also used up in the burning of fuels and in the formation of oxides of nitrogen. Yet, the amount of oxygen in the air remains more or less constant. This is because green plants return oxygen to the atmosphere by the process of photosynthesis. This circulation of oxygen is called the oxygen cycle. If the quantity of oxygen is increased in the air, the oxygen balance would get disturbed.

(c) Nitrogen is mostly found in its free state in the atmosphere. It is used by living beings

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to make proteins. It does not support combustion. Nitrogen is essential for the growth of plants and animals. Nitrogen in

the form of nitrates is absorbed by plants as nutrients. If it is reduced to half, the nitrogen balance would get disturbed.

Answers to Assessment 1

A. 1. (a) 2. (b) 3. (d) 4. (d)

5. (b)

B Defi ne the following words.

1. Rapid combustion- The type of oxidation in which oxygen reacts very quickly with a substance with the release of heat and light energy is called rapid oxidation. Rapid oxidation is also called combustion.

2. Catalyst - A catalyst is a chemical substance that alters the rate of a chemical reaction without undergoing any change in its chemical composition.

3. Silicones - Silicon forms polymers called silicones which find various uses such as making food wrappers, bulletproof materials, waterproof clothing and artificial implants for our body. It is also used to make non-stick cooking utensils.

4. Galvanisation - Galvanization is the process of applying a protective zinc coating to steel or iron, to prevent rusting/corrosion. The most common method is hot-dip galvanizing, in which parts are submerged in a bath of molten zinc.

5. Decomposition reaction - In a decomposition reaction, a compound breaks up into simpler compounds or elements. This requires heat. For example, on heating, calcium carbonate decomposes into calcium oxide and carbon dioxide.

C Hydrogen the lightest gas

Sulphur used to make insecticides

Helium used in weeding

Oxygen necessary for combustion

Diamond used for cutting glass

D 1. Mercury

2. Animal charcoal

3. Ammonia

4. Hydrated ferric oxide

5. Reactants

E 1. In a decomposition reaction, a compound breaks up into simpler compounds or elements and this requires heat. These are also known as a synthesis reaction. On the other hand, in a combination reaction, two or more reactants combine to form a more complex product.

2. They are the fuel types but are quite different and they are also made in different ways. So the difference is that coal is a mineral and charcoal is the ultimate form of burnt wood.

Coal is a hard, black, rock-like solid. It is mined out of the ground like any other rocks or minerals are mined.

Charcoal is a black substance made by burning of wood in a partial manner which creates a black substance which is used as fuel for purposes like heating, cooking among other things.

Charcoal is a favorite because it is easily gotten from trees unlike Coal which has to be mined.

Charcoal also burns far much cleaner, hotter and is consistent unlike Coal which is less cleaner and is not consistent because of the way it is gotten.

3. Nitrogen is a colourless and odourless gas. It is an element used by plants for fixation in the form of nitrates. We use atmospheric nitrogen to prepare ammonia which, in turn, is used to prepare nitrogen fertilizers. Liquid nitrogen is used to preserve food, tissues and human organs. Argon is also

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the most abundant of the truly inert or “rare” gases. It is filled in electric bulbs so as to provide an inert atmosphere to the heating fi lament. It improves the quality and life of the bulb.

4. Respiration takes place in living cells only. It is the oxidation of food with the liberation of energy and carried out with the help of enzymes.

On the other hand, Combustion does not take place in the living cell. The substances are oxidized spontaneously with sudden release of energy. Energy released is dissipated as heat. Enzymes are not involved in this process.

F 1. False

2. False

3. True

4. True (in the form of NH3)

5. True

G 1. The various characteristics of chemical reactions are-

Change of color, evolution of gas, formation of precipitate, occurrence of fire, evolution of heat or cooling e�ect.

2. The process of the slow eating away of a metal due to the attack of atmospheric gases and moisture on its surface is called corrosion.

Metal + Oxygen + Moisture → Rust .This process is also known as slow oxidation

3. The properties of metalloids are as follows.

– They have metallic lustre.

– They are brittle.

– Metalloids such as silicon and germanium are semiconductors under certain conditions.

– They form alloys with metals.

– They are solids at room temperature.

– They are similar to non-metals in their chemical behaviour.

4. Atmospheric oxygen and moisture are the conditions for rusting to occur.

5. Gold has lustre, that is, shine. It is malleable and ductile so used in making jewellery.

6. Burning of fossil fuels is hazardous for the lives of people. Therefore, we should switch over to the alternative fuels to avoid air pollution. For example, solar energy, hydropower and wind energy are non-polluting sources of energy.

– We should also reduce the usage of personal vehicles and use public transport or carpools.

– Plant more trees, so that carbon dioxide gets used up by plants for photosynthesis.

7. Air contains 78% nitrogen (major component of air), 21% oxygen, 0.04% carbon dioxide, 0.9% argon and 0.16% other gases. Apart from these, air also contains water vapour. The concentration of the components of air is not fixed at all places; hence, it is a mixture.

H 1. To show the evolution of gas during a chemical reaction , add a pinch of baking soda to the test tube containing 5 ml acetic acid. Fit it in the cork in which the glass tube is inserted. Pass the other end of the glass tube into the boiling tube containing lime water. Heat the boiling tube containing baking soda. Gas is evolved with strong e�ervescence and the lime water turns milky. Thus we conclude that when baking soda reacts with acetic acid, carbon dioxide (gas) and water are formed.

2. The properties of metals are-

– Metals have lustre, that is, they shine.

– They are malleable and ductile.

– Metals generally have high melting and high boiling points.

– They are good conductors of heat and electricity.

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Properties of non metals are -

– They don’t have lustre, so they look dull.

– They are non-malleable and non ductile.

– They are bad conductors of heat and electricity

– Non-metals exist as solids or gases.

3. (a) Coal. Coal is a hard, black, rock-like solid. Coal is mostly used as a fuel in homes and industries. It is used in thermal power plants for generating energy. Many trains still run on coal. It is used in the pharmaceutical and textile sectors, as a source of synthetic chemicals.

(b) Diamond. Diamond is one of the crystalline forms of carbon. It shines brilliantly and is the hardest naturally occurring substance known to us. Pure diamond is used as a precious gem in jewellery. Impure diamond is used for cutting glass, grinding hard substances and in drilling heads.

(c) Graphite. Graphite is a crystalline, black, lustrous substance. It is soft and soapy and turns paper black. Graphite is used in the lead of pencils, as electrode material in electrolytic cells because it is a good conductor of electricity, as a solid lubricant for maintaining machine parts , for making heat-resistant crucibles for keeping molten metals because of its high melting point and good conductivity of heat, in nuclear reactors (to control the speed of nuclear reactions).

4. The most convenient method to prepare oxygen in the laboratory is by the decomposition of hydrogen peroxide, as it does not involve heating. Hydrogen peroxide decomposes into water and oxygen on adding manganese dioxide.

Manganese dioxide acts as a catalyst. Manganese dioxide initiates and speed up the decomposition of hydrogen peroxide to release oxygen.

Hydrogen peroxide → Water + Oxygen

2H2O2 → 2H2O + O2

5. Undesirable gases and particles present in the air that can have harmful effects on plants and animals are called air pollutants. Some common pollutants in air are smoke, dust, soot, oxides of carbon, sulphur and nitrogen.

The following are some harmful e�ects of air pollutants.

– An increased concentration of carbon dioxide in the atmosphere causes the greenhouse e�ect, thus causing global warming.

– Carbon monoxide causes headaches, dizziness, weakness, upset stomachs, vomiting, and chest pain.

– Gases such as sulphur dioxide and nitrogen oxide cause acid rain.

– Sulphur dioxide, carbon dioxide or oxides of nitrogen cause respiratory disorders.

– Chlorofluorocarbons react with ozone and deplete the ozone layer, leading to an increase in the rate of skin cancer. This is because the ozone layer blocks out certain cancer-causing rays in the sunlight.

– Dust particles and carbon form suspended particulate matter (SPM). When inhaled, SPM may cause respiratory disorders.

I. 1. Carbon cycle in nature

2. Carbon dioxide is used in photosynthesis, and released in respiration.

3. Oxygen.

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WORKSHEET 1

1. a. Symbiosis

b. Air pollutants

c. carbogen

d. Sodium bicarbonate

e. oxygen

2. The natural cycle of consumption of oxygen by respiration and burning, and its release by photosynthesis is called the oxygen cycle. Thus, oxygen is naturally balanced.

3. Baking powder, a mixture of baking soda and potassium hydrogen tartarate is used for baking purposes because it produces CO2 in the dough causing it to rise.

4. Dry ice is the solid form of CO2. It acts as a good refrigerant and is used to keep food items, such as vegetables and meat, frozen.

WORKSHEET 2

1. Rusting of iron and respiration are examples of slow oxidation.

2. Oxygen mixed with acetylene produces a fl ame with very high temperature. This is called oxy-acetylene fl ame and is used in welding torches.

3. A catalyst is a chemical substance that alters the rate of a chemical reaction without undergoing any change in its chemical composition.

e.g. Manganese dioxide is used as a catalyst in the decomposition of hydrogen peroxide to release oxygen.

4. When oxides of nitrogen and sulphur mix with rain water, they form HNO3 and H2SO4 respectively. Thus, the rain that contains acids such as sulphuric acid, carbonic acid and nitric acid is called acid rain. It is harmful to the crops and aquatic organisms. It corrodes metal structures and bridges.

CO2 is uses as fi re extinguisher as it does not support combustion.

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Total marks: 50Time: 3 hours

Model Test Paper 2

Section AA. Fill in the blanks. [5]

1. A chemical reaction is the change of a substance into a new one that has a _______________ chemical identity.

2. Most of the metals combine with oxygen to form _______________ .

3. Non-metals are not _______________. They produce a dull sound.

4. _______________, mixed with oxygen, is administered to asthma patients.

5. _______________ is a lustrous grey metalloid.

B. Multiple choice questions. [5]

1. A metal used in fi lling tooth cavities.

a. Ag and An b. Zn and Na c. K and Na d. None of the above

2. The gas used as a coolant in refrigerators

a. Nitrogen b. Oxygen c. Sulphur e. Helium

3. In the equation, 2Mg + O2 →heat 2MgO, the reactants are

a. MgO and O2 b. Only MgO c. Mg and O2 d. Mg, O2 and MgO

4. Metals that have low melting and boiling points

a. Na and K b. Mg and K c. Pb and Mg d. Au and Mg

5. Mercury is used as a thermometric liquid because it _________________ even for a very small increase in temperature.

a. expands b. contracts

c. remains the same d. fi rst contracts and then expands

C. State True or False. [5]

1. Magnesium and tin are used in fi re works.

2. Coke is an impure form of amorphous carbon.

3. Sand is the principal ingredient of glass.

4. Oxygen was fi rst discovered by Joseph Priestly in 1774.

5. Green plants use nitrogen to carry out photosynthesis.

D. Name the following. [4]

1. Rain that contains acids such as sulphuric acid, carbonic acid and nitric acid.

2. Relationship where plants and bacteria are mutually benefi tted.

3. Representation of a chemical reaction using symbols and formulae.

4. Reduction of nitrates to gaseous N2 (free nitrogen) by anaerobic bacteria.

5. A chemical substance that alters the rate of a chemical reaction without undergoing any chemical change itself.

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Section BA. Defi ne the following. [3]

1. Ductility

2. Ammonifi cation

3. Air pollution

B. Short answer questions. [12]

1. How is CO2 useful in medical fi eld?

2. What happens when Potassium Chlorate is heated? Name the catalyst used.

3. Differentiate between:

a. Galvanisation and tinning

b. Chemical equation and word equation

C. Long answer questions. [12]

1. How will you demonstrate the evolution of heat during a chemical reaction?

2. Write the properties of non-metals.What type of oxides do they form?

3. Write the chemical equation for the following word equations:

a. Carbon dioxide + Water → Carbonic acid

b. Carbon + Oxygen → Carbon dioxide

c. Hydrogen + Chlorine → Hydrogen chloride

d. Sodium + Chlorine → Sodium chloride

4.

a. Which process is shown in the above picture?

b. What are the conditions required for the above process to occur?

c. What is the reddish-brown layer formed on metal known as?

d. Mention two methods to prevent the formation of this layer.

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Answers to Model Test Paper 1

Section A

A 1. element; compound

2. together

3. temporary; reverse

4. increases

5. CO2 and heat

B 1. irreversible and endothermic

2. exothermic

3. electrons

4. one

5. NH4+

C 1. True 2. True 3. False

4. True 5. False

D 1. Groups 2. Neutrons

3. Alloy 4. Respiration

Section B

A 1. A mixture containing oil and water is separated using this method. There is a stopcock at the tail end of the funnel. A liquid mixture when poured into the funnel and allowed to stand, separates into di�erent layers. Each layer can be drawn out into di�erent containers by opening the stopcock.

2. Melting, or fusion, is a physical process that results in the state transition of a substance from a solid to a liquid. It occurs at a specific temperature called the melting point. It is a fast, reversible, endothermic.

3. A change that releases energy in the form of heat is called an exothermic change. The reactants have more energy than the products. The container in which an exothermic change is taking place will be warm.

B 1. These are all physical changes because they are temporary changes and can be easily reversed.

2. Salt is extracted from seawater by the process of evaporation. Seawater is collected in shallow pans or ponds known as salt pans and is allowed to evaporate in the sun’s heat. This process takes a few days and, finally, it leaves behind salt crystals, which contain some impurities that are removed by further purification methods. This method of removing salt from seawater is called desalination.

3. An ionic bond is formed by the transfer of one or more electrons from a metal to a non-metal. It is also known as the attractive force that binds oppositely charged ions together. Compounds of metals and non-metals are formed due to ionic bonds. On the other hand, compounds formed by the sharing of electrons between elements are called covalent compounds and the chemical bond formed by the sharing of electrons between the atoms is called a covalent bond.

4. Valency is the combining capacity of an atom of an element. It is determined by the number of electrons an atom gains or loses. It is always represented by a whole number. An oxygen atom can combine with two hydrogen atoms to form one molecule of water. Hence, its valency is 2.

The number of atoms present in a molecule of an element is called atomicity. Two or more atoms combine to form a molecule. For example, gases like hydrogen, oxygen and nitrogen have two atoms in their molecules. They are called diatomic molecules or their atomicity is two.

C. 1. Fractional distillation is used to separate mixtures comprising miscible liquids like alcohol and water. It is based on the di�erence in the boiling points of the liquids.

The distillation flask is fi tted with a fractionating column, which is connected to the condenser. There is a thermometer

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inserted into the fractionating column. The temperature across the column varies; it is higher at the bottom and lower at the top. The vapours of the component with a higher boiling point condense at the bottom of the column. The liquid with the lower boiling point vapourises first and rise up. The vapours of the other liquid coming along in small quantities will be condensed and sent back into the flask by the fractionating column. The vapours of the liquid with the low boiling point rise and are condensed in the condenser. This liquid is then collected in a container. After the entire first component is removed, the temperature starts rising till the boiling point of the second liquid is reached. Its vapours are also condensed and collected in a di�erent container.

2. The sugar crystals melt on heating. The colour of the liquid sugar changes to brown. On further heating, this brown sugar is converted into black char. Thus, Melting of sugar is a physical change, as we can get back sugar. The formation of char is a chemical change because sugar loses its water, leaving behind its carbon contents in the form of char. This change is permanent and new substance is formed.

3. a. Sublimation of AlCl3

b. Sublimation is a change from the solid

to the vapour state without melting.

c. Ammonium chloride vaporises and gets deposited near the tail end of the funnel.

d. Salt is left in the china dish.

e. Magnesium Oxide


Mg O


+2 –2


Mg2 O2


1 : 1


MgO

Calcium Chloride


Ca Cl


+2 –1


Ca1 Cl2


1 : 2


CaCl2

Answers to Model Test Paper 2

Section A

A 1. different 2. oxides 3. sonorous

4. helium 5. antimony

B 1. Ag and Au 2. Nitrogen

3. Mg and O2 4. Na & K

5. expands

C 1. True 2. False 3. True

4. True 5. False

4. 1. Acid rain

2. Symbiosis

3. Chemical equation

4. Denitrifi cation

5. Catalyst

Section B

A 1. The property of metals by virtue of which they can be drawn into wires without breaking is called ductility. Gold and silver are the two most ductile metals known.

2. When plants and animals die, bacteria

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break down nitrogen nutrients (proteins) to release ammonium salts and ammonia. This conversion process is called ammonification.

3. The release of air pollutants into the air through unwanted human activities or by natural processes is called air pollution.

B 1. Oxygen mixed with 5% carbon dioxide is called carbogen, which helps in treating gas poisoning, su�ocation and shocks.

2. When Potassium chlorate is heated, it undergoes thermal decomposition, giving out oxygen.

2KClO3 MnO2→ 2KCl + 3O2

Manganese dioxide is used as a catalyst to speed up the reaction.

3. a. Galvanisation is the coating of metal with zinc to prevent it from corrosion, whereas Tinning is the coating with tin.

b. Chemical equation is the representation of a chemical reaction using symbols and formulae whereas word equation is the representation of a chemical reaction using the name of the chemicals.

Equation: Magnesium + Oxygen → Magnesium oxide

Chemical equation:

2Mg + O2 → 2MgO

C 1. Take 6-8 cm of magnesium ribbon rubbed with sand paper to remove oxide coating. Light up the burner and adjust the flame to light blue.

– Hold one end of the magnesium ribbon with tongs and place the other end of the ribbon in the flame until it ignites.

– Never look directly at the burning ribbon as the burning magnesium yields brilliant light and intense heat.

It is observed that the magnesium ribbon burns with a dazzling white flame and changes into a white powder, giving out heat. This powder is magnesium oxide.

2. Non-metals exist as solids or gases

– They don’t have lustre, so they look dull

– They are non-malleable and non ductile. On heating, they do not melt but burn.

– Non-metals are generally soft. Diamond is an exception, as it is the hardest known substance.

– Most non-metals form acidic oxides.

3. a. Chemical equation:

CO2 + H2O → H2CO3

b. Chemical equation: C + O2 → CO2

c. Chemical equation: H2 + Cl2 → 2HCl

d. Chemical equation:

2Na + Cl2 → 2NaCl

4. a. Rusting

b. It occurs in the presence of oxygen and moisture of the atmosphere.

c, Reddish brown and flaky layer is called rust. Rust is actually iron oxide (Fe2O3.nH2O).

d. Rusting of metals can be prevented by:

– Applying oil or grease

– Applying paint

– Galvanisation (coating with zinc)

– Tinning (coating with tin)

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