Q1(MCQ): A body which emits light by itself is (a) Luminous body (b) Non luminous body (c) Shadow (d) Opaque body Q2: Fill in the Blanks (i) Object like the sun that give out or emit light of their own are called ______________ Object. (ii) Light travels in ______________ line. (iii) _________________ are formed when an opaque object comes in the path of light (iv) Polished surfaces produce ____________________ reflection which causes glare in our eyes. (v) ______________________objects allow light to pass through them completely. (vi) ______________________ objects do not allow the light to pass through it at all. (vii) _______________________ objects allow the light to pass through them partially. (viii) Stars are ________________ sources of light. (ix) Shadows are of two kinds named _________ and ____________. Answer: (i) luminuous(ii) straight(iii) Shadows(iv) regular(v) Transparent(vi) Opaque(vii) Translucent(viii) natural(ix) umbra, penumbra Q4: What are luminous objects? Give two examples. Answer: Objects that emit their own light are called luminous objects. Stars, Sun, candle, torch are examples of luminous objects. The light emitted by luminous objects enables us to see things. Q5: What are non-luminous objects? Give two examples. Answer: The objects which do not emit light are called non-luminous objects. Table, chair, trees, moon are examples of non-luminous objects. Q6: To see an object what do we need? Answer: To see any non-luminous object we need the following:

source of light an object eyes

When a light emitting from luminous body falls on a non-luminous object and is reflected. This reflected light reaches our eyes enables us to see that object. Q7: Moon appears bright at night. Is it a luminous or non-luminous body? Answer: Moon is a non-luminous body, it does not emit its own light. It reflects the sun light that falls on it. Q8(NCERT): Classify the objects or materials given below as opaque, transparent or translucent and luminous or non-luminous: Air, water, a piece of rock, a sheet of aluminium, a mirror, a wooden board, a sheet of polythene, a CD, smoke, a sheet of plane glass, fog, a piece of red hot iron, an umbrella, a lighted fluorescent tube, a wall, a sheet of carbon paper, the flame of a gas burner, a sheet of cardboard, a lighted torch, a sheet of cellophane, a wire mesh, kerosene stove, sun, firefly, moon. Answer:

OBJECT

TRANSPARENT

/TRANSLUCENT

/OPAQUE

LUMINOUS/

NON-LUMINOUS

AIR, WATER TRANSPARENT NON-LUMINOUS

PIECE OF ROCK, OPAQUE NON-LUMINOUS

OBJECT

TRANSPARENT

/TRANSLUCENT

/OPAQUE

LUMINOUS/

NON-LUMINOUS

SHEET OF ALUMINIUM,

MIRROR, WOODEN BOARD,

CD, UMBRELLA, WALL,

SHEET OF CARBON PAPER,

SHEET OF CARDBOARD,

MOON

A SHEET OF POLYTHENE,

SMOKE, FOG,

SHEET OF CELLOPHONE,

WIRE-MESH,

TRANSLUCENT NON-LUMINOUS

A PIECE OF RED HOT IRON,

LIGHT FLUORESCENT TUBE,

LIGHTED TORCH,

KEROSENE STOVE,

SUN, FIREFLY

OPAQUE LUMINOUS

FLAME OF GAS BURNER TRANSLUCENT LUMINOUS Q9: How shadows are formed? What are different types of shadows? Answer: When some objects are placed in the path of light, a dark portion is formed on the opposite side of the objects. This dark portion is called shadow.There are two types of shadows:

umbra penumbra. The umbra is formed where no light reaches and it is a dark, black shadow.

The penumbra is formed where some light reaches and it is grey in color. Q10: Do all objects cast shadow? Answer: No. Only opaque objects cast shadow. Q11: Which type of surfaces produce clear images? Answer: Polished and plane reflecting surfaces produce clear image. Q13(NCERT): In a completely dark room, if you hold up a mirror in front of you, will you see a reflection of

yourself in the mirror?

Answer: To see the reflection, source of light is required We can only in the presence of light.

Q14: What is lateral inversion?

Answer: When an object is placed in front of a plane mirror, the right side of the object appears to be on the left side of image

and left side of the object appears to be on the right side of image. This is called lateral inversion.

Q15: State the principle behind the working of a pinhole camera.

Answer: Light travels in straight line. This principle is behind the working of pinhole camera.

Q16: What are transparent objects?

Q17: What are opaque objects?

Q18: What are translucent objects?

Q19: What types of objects do not cast shadows?

Answer: Transparent objects and few translucent objects, because light passes through them.

Q20(MCQ): The image formed in pinhole camera is

(a) Inverted

(b) Erect

(c) Both a and b

(d) None of the above

Shadow: When light falls on an opaque object, a dark patch is formed on the other side of the object; if a screen is

present on the other side. Three things are required for formation of shadow, viz. a source of light, an object and a

screen. The size of shadow depends on the distance of source of light and on the angle at which the light rays fall

on the object.If the source of light is closer to the object, a larger shadow is formed than when the source of light

is far from the object. If the angle of incident light is smaller, the shadow is longer. On the other hand, if the angle

of incident light is bigger, the shadow is smaller. This explains, why our shadows are longer in the morning and

evening and smaller in the noon. When light falls on a shiny surface, the light rays bounce back. This

phenomenon is called reflection of light. Mirror is a surface from which most of the incident light gets reflected.I)

Fill in the blanks:1. __________________ helps us to see objects.(Non luminous)

2. Objects that give out or emit light of their own are called _________(Luminous)

3. ____________objects allow light to pass through them completely. (Transparent)

4. ____________objects do not allow the light to pass through it at all. (Opaque) 5. ____________objects allow the light to pass through them partially.(Translucent)

6. Light travels in a _____________.(Straight path)

7. _____are formed when an opaque object comes in the path of light.(Shadow)

8. __________ and ___________ objects are essential for the formation of shadows on a screen.(Light,Screen)

9. A shadow cast by the heavenly bodies is called an ________( Eclipse)

10. Images formed by a pin –hole camera are ______________(inverted)

11. We can see _________ of the object in the mirror. (Image)

12. All sources of light can be classified into two categories _______ and ______

(Natural,Artificial)

13. Moon and planets do not emit light of their own, but they reflect the light of the ____ falling on them. (sun)

14. A non-luminous body can be made luminous by _____ it. (lighting )

15. A source of light which is of the size of the head of a common pin is called ____________________ source of light. (Pointed)

(II) Choose the correct Answer : 1. [Mirror/glass] helps to change the direction of light that falls on it. 2. Images are [ same / different] from the shadow. 3. Torch bulb is [ luminous / non – luminous object] 4. [opaque/transparent] objects cast shadows. 5. Coloured objects form [coloured / dark] shadows. III) Define : 1. Opaque objects 2. Shadows 3. Reflection of light IV) Distinguish between 1. Transparent and translucent objects 2. Luminous and non luminous objects. (V) Answer the following questions : 1. How are shadows formed ? 2. What is meant by reflection of light ? 3. Explain with the help of an activity that light travels in a straight line.

ass-6th Light

I. Answer these questions

1) Define Light, Reflection of light , Shadow , Mirror, Incandescence

2) What luminous body give example of natural and manmade luminous

body

3) How does the moon differ from the sun in the way it shines?

4) What is rectilinear propagation of light? How can you show that?

5) What are transparent, translucent and opaque materials? Give three

example of each

6) What is the reason of shadow formation and what are the factors that

affect its size

7) What do you mean by inverted image Explain with the help of burning

candle.

8) What are regular and irregular reflection

II. Objectives

a) Emission of light by a body that is not hot -------------.

Brain Teaser Quest Time For Class 6th Light, Shadows And Reflections

1. How can you determine the length of the shadow of an object?Ans: We cannot correctly determine the length of the

shadow of an object. The length of shadow depends on the angle at which light fall on a body and varies at different time of

day. This principal helps us to make sun dial for calculating time.

2. How can you make sundial at home?

Ans: We take a pencil and place it into the whole of the empty spool of thread.This work as sundial.

Making the Sundial:

1. Using the pencil, poke a hole on the side of the paper / Styrofoam cup pproximately 2 inches below its top (rim).

2. Place the pebbles in the cup so to give it some weight and hold it upright.3. Cover the cup with the plastic lid.

4. Put the straw through the hole on the side of the cup and its lid while letting about half an inch of the straw stick out

from the side.

4. Secure the straw to the cup by taping it down on the side.

3. How do you make a pinhole camera?

Ans: Take two rectangular boxes that fit into one another without leaving any gap.

Cut open one side of each box.

Make a small hole in the larger box at the centre of the closed end opposite the side that has been cut open.

Cut a square of side five centimetres in the smaller box in the closed end opposite the side that has been cut open. Cover

this square with tracing paper.

Finally, slide the smaller box into the larger box,

ensuring that the pinhole and the tracing paper are in line with one another, but at the opposite ends.

Slide the smaller box to adjust the focus so that you can capture the image of any object you want.

Look through the open face of the smaller box. you can see the image of the object and inverted on the screen.

4. Explain with the help of an activity that light travels in a straight line.

Ans: Place a candle at one corner of the room

Now look through the pipe, you can see the candle

Now bend the pipe and look. you cannot see the candle

This shows that light travel in straight line called rectilinear propagation of light

5. What is meant by reflection of light ? Ans: When light fall on smooth or rough surface return back after hitting these

surface this phenomenon is called reflection of light

6. What types of image form in pinhole camera?Ans: Inverted (upside-down)

7. Why is the image formed by a pinhole camera upside down?Ans: It is because there is no refracting or reflecting optical

element to change the path of the rays of light.8. What happens if we place an opaque object in coloured light?The colour

of light will not affect the shadow, because shadow is the dark patch formed when an object obstructs the path of light9.

On a sunny day, does a bird or an aeroplane flying high in the sky cast its shadow on the ground? Under what circumstances

can we see their shadow on the ground?Ans: when the bird is flying very low close to the ground.

10. You are given a transparent glass sheet. Suggest any two ways to make it translucent without breaking it

Ans:

(i) By applying oil, grease, butter on it or pasting a butter paper on it.

(ii) Grinding (rubbing) the surface of the glass by any abrasive material.

11. Suggest a situation where we obtain more than one shadow of an object at a time.

Ans: We can obtain more than one shadow of an object if light from more than one source falls on it. [For example during a

match being played in a stadium, multiple shadows of players are seen].

12. Three identical towels of red, blue and green colour are hanging on a clothes line in the sun. What would be the colour

of shadows of these towels?

Ans: The colour of shadows of all three towels will be the same

13. Using a pinhole camera a student observes the image of two of his friends, standing in sunlight, wearing yellow and red

shirt respectively. What will be the colours of the shirts in the image?

Ans: The colours of the image of the shirts will be the same as the colour of the shirt.

14. A football match is being played at night in a stadium with flood lights ON. You can see the shadow of a football kept at

the ground but cannot see its shadow when it is kicked high in the air. Explain.

Ans: We can see the shadow of football lying on the ground because the ground acts as a screen for it. However, when the

football is kicked high, the ground, which is acting as a screen is away from the football, hence no shadow of the football

will be formed on the ground

15. . A student had a ball, a screen and a torch in working condition. He tried to form a shadow of the ball on the screen by

placing them at different positions. Sometimes the shadow was not obtained. Explain.Ans: Possible reasons (i) The screen

away from the ball.(ii) The beam of light from the torch is falling parallel to the screen on the ball.(iii) The torch is kept away

from the ball.

Quest Time CBSE NCERT VI Chapter: Electric currents, Circuits, Cell.

1. From where do we get electricity?

Ans: A power station provides electricity. If the supply of electricity may fail we use other sources of energy

like torch at home.

2. Why does the bulb glow switch on a torch?

Ans: It glows because the electric energy stored in the electric cells is converted into light energy by the bulb.

Hence Electric cell is a source of light energy.

3. Name the scientist who developed first Cell?

Ans: The first electric cell was developed by an Italian scientist Luigi Galvani and then improved by Alessandro

Volta.

4. Name a cell that is recharged by Light of The Sun?

Ans: alkali cells and solar cells. Solar cells convert light energy into electric energy.

5. What is an electric cell?

Ans: An electric cell is a device which converts chemical energy into electric energy.

The cell has two different metal plates – one is the positive terminal and the other is the negative terminal.

These plates are kept inside a chemical called electrolyte. One of the chemical is white in color NH4Cl

(Ammonium chloride) and other is a black powder MnO2 (manganese dioxide).

6. What is an electric current?

Ans: Electric current is the flow of electrons or charge. The cell is a source of electric current.

7. What is an electric circuit?

Ans: An electric circuit is the closed path along which electric current flows from the positive terminal to the

negative terminal of the battery.

A circuit generally has:

a) A source of electric current - a cell or battery(A group of cell called battery)

b) Connecting wires for carrying current.

c) A device which uses the electricity - a bulb ( Bulb is a device that convert electrical energy into light)

d) A key or a switch – A component of circuits that stop or allow the flow of current. When the current flows the

circuit is said to be closed circuit. When the current does not flow, the circuit is said to be open circuit.

8. How does a bulb glow and produce light?

Ans: Inside the bulb there is a thin coil of wire made of tungsten, called filament. It gets heated and glows when

the current flows. Hence, electric energy is converted into light energy by a bulb.

9. If the filament inside the bulb is broken will the bulb glow? Why?

Ans: The circuit is not complete; current does not flow through the circuit and the bulb does not glow.

10. Why do we feel warm when we touch the lighted bulb?

Ans: The bulb also emits heat energy along with light.

Q1. Fill in the blanks :

(a) A device that is used to break an electric circuit is called _______________.

(b) An electric cell has _______________ terminals.

(c) Electric cell is a device which converts energy of chemicals into ________________energy.

(d) The electric energy which is supplied in our home comes from electric _______________houses.

(e) An electric ______________ is a continuous path along which the current flows.

(f) If the filament of a bulb breaks, it is said to be ______________.

(g) Rubber is a good example of electric ______________.

Answer:

(a) switch

(b) two terminals

(c) electrical

(d) poles nearby

(e) circuit

(f) fused

(g) insulator

Q2: Mark 'True' or 'False' for following statements:

Answer:

(a) Electric current can flow through metals. (✓ True)

(b) Instead of metal wires, a jute string can be used to make a circuit. (✗ False)

(c) Electric current can pass through a sheet of thermocol. (✗ False)

(d) When current flows through a circuit, the circuit is called open circuit. (✗ False)

(e) An electric bulb has one terminal (✗ False)

Q3: What is an electric cell?

Answer: An electric cell is a device which converts chemical

energy into electric energy. It has two different metal plates:

one is the positive terminal

the other is the negative terminal

These plates are kept inside a chemical called electrolyte, which stores the chemical energy.

Q4: Explain why the bulb would not glow in the arrangement shown in Fig. below:

Answer: The handle of the screw driver acts as an insulator. It does not allow current to flow and circuit is still

open. Therefore the bulb does not glow in the arrangement.

Q5: Complete the drawing shown in Fig below to indicate where the free ends of the two wires

should be joined to make the bulb glow.

Answer: The bulb will glow when circuit is complete i.e. one end of the wire from clip is connected to positive

terminal. Second end of the wire from the clip is connected to the bulb.

Q6: Would the bulb glow after completing the circuit shown in Fig. above (Q5) if instead of safety pin we use an

eraser?

Answer: No. An eraser is made of rubber which is a poor conductor of electricity. Therefore, bulb would not

glow.

Q7: What is the purpose of using an electric switch? Name some electrical gadgets that have switches built

into them.

Answer: An electric switch is a simple device which is used to break or complete an electric circuit. Switches

are widely used in different electrical or electronic gadgets. A few are:

Toggle switches used in houses.

Pushbutton switches used in microwaves, water pump, mixer etc.

Joystick switches used in Video Games Controllers, Toy Cars etc.

Q8: Do all materials allow electricity to pass through them?

Answer: No.

Q9: Would the bulb glow in the circuit shown in Fig. below?

Answer: The electric bulb has two terminals, both the wires are connected to the one terminal of the bulb.

Current will not flow from the bulb and it will not glow. To make glow, the two wires should be connected to

each terminal of the bulb, as shown below.

Q10: What are the components of an electric circuit?

Answer: In general a circuit has the following components:

1. A cell or battery : source of electricity.

2. Connecting wires

3. Bulb or electric device

4. Key or switch.

Q11: How many terminals are there in an electric cell?

Answer:An electric cell has two terminals; positive and negative. The metal cap is the positive terminal of the

cell. The metal disc is the negative terminal.

Q12: Using the "conduction tester" on an object it was found that the bulb begins to

glow. Is that object a conductor or an insulator? Explain.

Answer:Since the object allows current to flow and completes the circuit. Due to this bulb begins to glow. It

means the object is a conductor of electricity.

Q13: What are conductors? Give examples.

Answer: The materials which allow electric current to pass through them are called conductors.

Examples are: All metals (copper, silver, iron) are good conductors. Water usually contains impurities and

traces of salt is also a good conductor. Human body is also a conductor.

Q14: What are insulators? Give examples.

Answer: Materials that do not allow electricity to pass through them are called insulators. Insulators oppose

electric current and so they are used as protection from the dangerous effects of electricity.

Examples of insulators are glass, air, plastic, cotton, thermocol, wood and rubber.

Never touch electric circuit boards

(image credits: Phillip Martin)

Q15: Why should an electrician use rubber gloves while repairing an electric switch at your home? Explain

Answer: Electricians need to touch copper wires or they may accidentally come in contact with live wires

(wires carrying current). Since human body is also a good conductor, electricians may get shock. Rubber is

good insulator. To prevent themselves from electric shock or flow of current, the electricians use rubber gloves

while repairing.

Q16: The handles of the tools like screwdrivers and pliers used by electricians for repair work usually have

plastic or rubber covers on them. Can you explain why?

Answer: Without the help of insulators, the use of electrical tools like pliers and screwdrivers is impossible.

Wood and plastics are insulators and help in avoiding direct contact with electric current. When electricians

touch these live electric wires with the tools covered with plastic and wood, current does not flow in their body

avoid any accident.

Q21: What will happen if you join the two terminals of a cell without connecting them through a switch or a

bulb?

Answer: By doing so, the chemicals in the electric cell get used up very fast and the cell will no longer produce

electric current. It will stop working.

Q22: Why do an electric bulb and an electric cell have two terminals?

Answer: Because the two terminals (positive and negative) can be connected to form an electric circuit.

Q23: Is Distilled water a conductor of electricity?

Answer: No. The distilled water is the purest water and it acts as an insulator. Therefore, it used in batteries as

an insulator. The tap water, sea water contains some traces of salts and impurities, due to which it becomes

good conductor of electricity.

For Questions 1 to 10 visit link: Class VI Physics By Jsunil

11. What conductors and insulators?Ans: The materials which allow electric current to pass through them are

called conductors. Examples: All metals like Copper,Iron, Silver and Human body.

INSULATORS: The materials which do not allow electric current to pass through them are called bad

conductors or insulators. Examples: Plastic, Wood, Rubber and Glass.12. Why do electricians wear rubber

gloves and shoes while at work?Ans: It because rubber gloves are an insulator and prevent current to flow in body .Thus

saves electricians from electric shock.13. What do you mean by heating effect of current?Ans: Electric current is actually the

flow of electrons in the conducting material. Now,when these electrons flow, they encounter some resistance in the

structure of the conductor, which accounts for the "resistance" of materials. As the electrons collide or vibrate, heat energy

is generated depending on the resistance of the conductor. This overall effect is called heating effect of electrical current.

14. Name a few appliances that work on the heating effect of current?

Ans: Electric kettle: Used for boiling water to make tea or coffee

Electric bulb : The filament of the bulb is heated when current flows and becomes white hot to emit brilliant light.

Electric iron: Used for ironing or pressing clothes.

Electric toaster: Used to toast bread

15. Name a special material used for heating component?Ans: It is a coil of wire made of a special material called nichrome

which becomes very hot when current is passed. This heat is used to cook food (as in an electric stove), heat water ( as in an

electric kettle, electric heater) etc16. Name the elements used to make an alloy nichrome?Ans: Nickel and Chromium.17

Why are electric wire are made up of copper through silver is best conductor?Ans: Silver is an expensive metal. Material

needed for conductor should be cheap as wires are used extensively in almost every appliance. That is why silver can't be

used to make wires.18. Why do you apply a tap over the joint of electric wire?Ans: Tap is an insulator and save us from

electric shock.19. If you connect a battery cell to the bulb through a wooden stick, the will not glow. Why?Ans: No, wooden

stick is an insulator that cannot conduct electricity20. Do not handle the electric appliances when your hands are wet.

Why?Ans: Tap water is good conductor and we may get electric shock.21. There will be three colored wires normally used

while connecting any electric equipment. What are those colors? Tell which coloured wire gives us electric shock?Ans: Red

(Live wire that give us electric shock), Black (Neutral) , and Green(Earth wire)

Choose the correct answer from the bracket.

1. In electric bells electric energy transform in to ____________energy.

(a)Mechanical energy (b) Magnetic energy (c) Sound energy (d) Light energy

2. ----------- is the positive terminal of an electric cell.

(a) Metal disc (b) Metal case (c) Metal cap (d) Metal base

3.The tiny coiled metal wire present inside the bulb is called____________.

(a) Element (b) Conductor (c) Filament (d) None of these

4. A combination of two or more electric cells is called___________.

(a) Electric circuit (b) Battery (c) Terminals (d) None of these

5. An electric bulb has ____________terminals.

(a) 3 (b) 4 (c) 1 (d) 2

6 ----------- is an electric device which easily closes or opens an electric circuit.

(a) Filament (b) Electric bulb (c) Electric switch (d) Electric cell

7. A material which does not allow the electricity to pass through it is___________.

(a) Conductor (b) Electric cell (c) Insulator (d) None of these.

8. Non metals like __________and __________are conductors of electricity.

(a)Wood and paper (b) Graphite and gas carbon (c) Glass and rubber (d) None of theses

9. A material which allows the electricity to pass through it is ________.

(a) conductor (b) Insulator (c) Transistor (d) None of these

10. Electric energy is produced from the____________ within the cell.

(a) Chemicals (b) Metals (c) Terminals (d) None of these

ANSWER KEY FOR ELECTRICITY & CIRCUITS

1)B 2)C 3)C 4)B 5)D 6)C 7)C 8)B 9)A 10)A

6th Electricity and Circuit MCQ'S based on NCERT/CBSE

11. A complete electric circuit is called______________ electric circuit.

(a) open

(b) Closed

(c) Complete

(d) None of these

12. ----------- is an insulator.

(a) Metal

(b) acid

(c) Rubber

(d) Salt solution

13.__________is a conductor.

(a) Wood

(b) Metal

(c) Paper

(d) Petrol

14. The electric current in a closed circuit always flows from the ___________terminal of the electric cell to the

______terminal.

(a) Negative to positive

(b) Positive to negative

(c) Positive to positive

(d) None of these

15. The base of an electric cell is its ___________terminal.

(a) Positive terminal

(b) Negative terminal

(c) Negative and positive

(d) None of these

16. The path along which electric current flows is_____________.

(a) Switch

(b) Electric current

(c) Electric circuit

(d) Electric cell

17. In _____________ electrical energy is transformed in to mechanical energy.

(a) Transistor

(b) Mixer grinder

(c) Electromagnet

(d) Electric bell.

18 ------------- is an energy which flows in an electric circuit.

(a)Electric circuit

(b) Electromagnet

(c) Electric current

(d) None of these.

19. The central _____________of an electric cell is always positive.

(a) Circuit

(b) Chemical

(c) Terminal

(d) None of these

20. Kerosene oil is an example of_______________.

(a) Conductors

(b) Insulators

(c) Electrolyte

(d) None of these

21. The cylindrical container of electric cell is made of ___________ _and acts as negative terminal.

(a) Copper

(b) Iron

(c) Zinc

(d) Silver

22. The wire which carries away the current to the power house is called____________.

(a) Earth wire

(b) Neutral wire

(c) Live wire

(d) None of these

23._____________ is a safety wire which protect us from electric shocks and electric fire.

(a) Earth wire

(b) Neutral wire

(c) Live wire

(d) None of these

24.____________ is a device which converts chemical energy in to electrical energy.

(a) Electrical bell

(b) Electric lift

(c) Electric cell

(d) Electric bulb

25. An electric cell has ____________terminals.

(a) 3

(b)4

(c)1

(d)2

ANSWER KEY FOR ELECTRICITY & CIRCUITS

(11)B (12)C (13)B (14)B (15)B

(16)C (17)B (18)C (19)C (20)B

(21)C (22)B (23)A (24)C (25)D

Q1: To walk through a waterlogged area, you usually shorten the length of your dress by folding it. Can this change be reversed? Answer: Yes, by unfolding the dress we can reverse this change. Q2: What are the various types of changes? Answer: Changes can be categorised as:

1. Slow and fast changes. 2. Reversible and irreversible changes. 3. Natural and man-made changes.

4. Periodic and non-periodic changes. 5. Physical and chemical changes.

Q3: You accidentally dropped your favourite toy and broke it. This is a change you did not want. Can this change be reversed? Answer: No, it cannot be reversed. It is an irreversible change. Q4: Some changes are listed in the following table. For each change, write in the blank column, whether the change can be reversed or not. Answer:

S. No. Change Can be reversed (Yes/ No)

1. The sawing of a piece of wood No

2. The melting of ice candy Yes

3. Dissolving sugar in water Yes

4. The cooking of food No

5. The ripening of a mango No

6. Souring of milk No

Q5: A drawing sheet changes when you draw a picture on it. Can you reverse this change? Answer: By erasing the drawing we can undo the change. For example, if we draw with the help of pencil, we can erase the drawing by using an eraser. Then, it is a reversible change. If we draw using a pen, we cannot erase the drawing. Then this change cannot be reversed. Q6: Bursting of cracker is a/an (a) reversible change (b) irreversible change (c) both reversible and irreversible (d) None of these Answer: (b) irreversible change Q7: Give examples to explain the difference between changes that can or cannot be reversed. Answer: Examples of Reversible Changes:

1. Melting of Ice into water. By freezing the water we can obtain ice again. 2. Folding a paper: By unfolding it, we can undo the change. 3. Hot milk to cold milk: By boiling milk, we can make it warm.

Example of Irreversible Changes:

1. Bursting a balloon. 2. Bud to flower 3. Cutting a paper into pieces

Q8: A thick coating of a paste of Plaster of Paris (POP) is applied over the bandage on a fractured bone. It becomes hard on drying to keep the fractured bone immobilised. Can the change in POP be reversed? Answer: When water is added to POP, it undergoes a chemical change. Its composition changes and is converted into another substance. Since it is a chemical change, it cannot be reversed.

Q9: A bag of cement lying in the open gets wet due to rain during the night. The next day the sun shines brightly. Do you think the changes, which have occurred in the cement, could be reversed? Answer: Due to water, cement hardens and its composition changes. Chemical changes, in general, are irreversible changes. Since it is a chemical change which cannot be reversed. Q10: What is a change? Answer: An object may undergo different look, composition, colour, position, size or shape due to some factor (external or internal). It is called a change. Change may be permanent or temporary. Q11: What are reversible changes? Give examples. Answer: Those changes which can be reversed to obtain original form by removing the cause of change are called reversible changes. Examples are:

melting of ice folding of paper evaporation of water

Q12: What is irreversible change? Give examples of irreversible change. Answer: The change in a substance which cannot be converted back to its original form is called irreversible change. Examples are:

burning of candle germination of seed bursting a balloon rusting of iron

Q13: What are slow and fast changes? Give examples also. Answer: Slow Changes: Changes that take place in longer duration i.e. few hours, days, months or years are called slow changes. Rusting of iron, growth of a baby, cooking of food, formation of coal etc. are examples of slow changes. Fast Changes: Changes that take place in a short duration (instant or nick of time) are called fast changes. Bursting of cracker, burning of paper etc. are examples of fast changes.

Q14: Identify from the following list of changes which are fast or slow. Also mention the duration that take place in happening the change. (a) Curdling of milk (b) Switching on light bulb in a dark room

(c) Formation of Petrol (d) Bursting a balloon (e) Turning of a child into an adult. (f) Burning a matchstick. Answer:

Changes Fast or Slow Duration

(a) Curdling of milk Slow Few hours

(b) Switching on light bulb in a dark room

Fast Instant

(c) Formation of Petrol Slow Million years

(d) Bursting a balloon Fast Instant

(e) Turning of a child into an adult. Slow 20 years

(f) Burning a matchstick. Fast less than a minute

Q15: What are periodic changes? Give examples of periodic changes. Answer: Those changes that occur at periodic intervals of time are called periodic changes. It is easy to periodic changes. Examples are:

Occurence of day and night Phases of moon Heartbeat

Q16: What are non-periodic changes? Also list two examples of non-periodic changes. Answer: Changes that do not occur at fixed intervals of time are termed as non-periodic changes. Such changes may be unpredictable. Examples are:

Earthquake Eruption of volcano An accident Sneezing

Q17: Differentiate between physical changes and chemical changes. Answer:

S.No. Physical Changes Chemical Chnages

1. Composition of substance remains same No new substances are formed.

Composition of substance changes. New substance os formed.

2. Change in shape, size or physical state occurs. Chemical properties of substance remain same.

New substance have new shape, physical state, different physical and chemical properties.

3. Physical changes are reversible. Chemical changes are irreversible.

4. e.g.: Melting of ice, expansion of iron on heating Burning of wood, rusting of iron

Q18: Explain how iron rim is fixed over a wooden wheel. Which type of change is associated with this activity?

Answer: The iron rim taken, has slightly smaller diameter than the wooden wheel The iron rim is fixed over the wooden wheel

by heating the rim first. On heating, the iron expands and becomes slightly bigger than the wheel. Once mounted, water is

poured on the rim to cool it down. On cooling the iron rim contracts to its original size. As a result it becomes fixed on the

wooden wheel.

Iron expands on heating and contracts on cooling, this physical change is associated with this activity.

Reversible and irreversible changes

1. Which substance can dissolve completely in water?

Sand

Flour

Salt

2. Which of these would NOT help salt crystals dissolve more quickly in water?

Stirring

Crushing the salt crystals to a fine powder

Lowering the temperature of the water

3. How could you separate a mixture of sand and salt?

Mix with water, filter, and then evaporate

Mix with water, evaporate and then filter

Filter and then mix with water

4. Will the steam from seawater be salty?

Yes

No

Some of the steam will be salty

5. Why is nothing left behind when distilled water is evaporated?

Because nothing was dissolved in it

Because anything that was dissolved has evaporated

Because nothing can dissolve in water

6. Which of these is a reversible change?

Metal rusting

Baking a cake

Freezing orange juice to make an ice lolly

7. Which of these is an irreversible change?

Candle wax melting

Candle wax burning

Ice cream melting

8. What new materials are formed when a candle burns?

Melted wax

Water and carbon dioxide and soot

No new materials are formed

9. How could you change burnt paper back into its original form?

Cool it

Scrape off the soot

You can't

10. Which of these statements is FALSE?

A reversible reaction produces new materials

Chemical changes are usually irreversible

Physical changes are usually reversible