class: 10th subject: science chapter 1: chemical …
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CLASS: 10TH
SUBJECT: SCIENCE
CHAPTER 1: CHEMICAL REACTIONS AND EQUATIONS (MCQ)
1. Which of the following reactions involves the combination of two elements ?
a. CaO + -> Ca
b. 4Na + -> 2N O
c. S + (1/2) ->
d. N + HCl -> N Cl
2. When hydrogen sulphide gas is passed through a blue solution of copper sulphate,
a black precipitate of copper sulphate is obtained and the sulphuric acid so
formed remains in the solution. The reaction is the example of
a. Combination reaction
b. Displacement reaction
c. Decomposition reaction
d. Double Displacement reaction
3. What happens when copper rod is dipped in iron sulphate solution
a. Copper displaces iron
b. Blue colour of copper sulphate solution is obtained
c. No reaction takes place
d. The solution becomes green in colour
4. A student added dilute HCl to a test tube containing zinc granules and made
following observations
a. The zinc surface becomes dull and black
b. A gas evolved which burnt with a pop sound
c. The solution remained colourless
d. The solution becomes green in colour
5. A dilute solution of sodium carbonate was added to two test tubes one containing
dilute HCl (a) and the other containing dilute NaOH (b). the correct observation
was:
a. a brown coloured gas liberated in test tube A
b. a brown coloured gas liberated in test tube B
c. a colourless gas liberated in test tube A
d. a colourless gas liberated in test tube B
6. A balanced chemical equation is in accordance with
a. Avogadro’s law
b. Law of multiple proportion
c. Law of conservation of mass
d. Law of gaseous volumes
7. The equation Cu + xHN -> Cu(N + yN + 2 O
The value of x and y are
a. 3 and 5
b. 8 and 6
c. 4 and 2
d. 7 and 1
8. When the gases sulphur dioxide and hydrogen sulphide mix in the presence of
water, the reaction is SO2 + 2H2S -> 2H2O + 3S. Here hydrogen sulphide is acting
as:
a. Oxidising agent
b. Reducing agent
c. Dehydrating agent
d. A catalyst
9. CuO + H2 ->H2O + Cu, reaction is an example of:
a. Redox reaction
b. Synthesis reaction
c. Neutralisation
d. Analysis reaction
10. A substance that oxidises itself and reduces other is known as
a. Oxidising agent
b. Reducing agent
c. Both of these
d. None of these
11. Fe2O3+ 2AI2O3 + 2Fe
This reaction is example of :
a. Combination reaction
b. Double displacement reaction
c. Decomposition reaction
d. Displacement reaction
12. White silver chloride in sunlight turns into
a. Grey
b. Yellow
c. Remains white
d. Red
13. Black and white photocopy uses
a. Decomposition of silver chloride
b. Decomposition of silver bromide
c. Both
d. None of these
14. When copper powder is heated it gets coated with
a. Black copper oxide
b. Yellow copper oxide
c. Red copper oxide
d. None of these
15. Combination of phosphorus and oxygen is an example of
a. Oxidation
b. Reduction
c. Rancidity
d. None of these
16. Rusting is an example of
a. Reduction
b. Ionization
c. Oxidation
d. Dissociation
17. Which of the following does not corrode when exposed to the atmosphere
a. Iron
b. Copper
c. Gold
d. Silver
18. Take about 1 g CaCO3 in a test tube. Heat it over the flame, when a colourless gas
comes out. The reaction is called
a. Decomposition reaction
b. Displacement reaction
c. Double decomposition reaction
d. Double displacement reaction
19. When P reacts with caustic soda, the products are PH3 and NaH2PO2 .This reaction
is an example of
a. Oxidation
b. Reduction
c. Redox
d. Neutralization
20. 2CuI-> Cu + CuI2 , the reaction is
a. Redox
b. Neutralization
c. Oxidation
d. Reduction
21. When copper is added to silver nitrate solution, a blue coloured solution is formed
after some time. It is because copper
a. It displaces silver from the solution
b. Forms a blue coloured complex with AgN
c. Is oxidised to C
d. Is reduced to C
22. A substance which oxidises itself and reduces other is
a. oxidising agent
b. reducing agent
c. both of these
d. none of these
22. When hydrogen sulphide gas is passed through a bluesolution of copper sulphate,
a black precipitate ofcopper sulphide is obtained and the sulphuric acid soformed
remains in the solution. The reaction is anexample of-
a. a combination reaction
b. a displacement reaction
c. a decomposition reaction
d. a double decomposition reaction
23 Which of the following is a physical change?
a. Formation of curd from milk
b. Ripening of fruits
c. Getting salt from sea water
d. Burning of wood
24. What happens when copper rod is dipped in ironsulphate solution?
a. Copper displaces iron
b. Bluecolour of copper sulphate solution is obtained
c. No reaction takes place
d. Reaction is exothermic
25. A dilute solution of sodium carbonate was added to two test tubes - one
containing dilHCl (a) and the other containing dilute NaOH(b). The
correctobservation was-
a. a brown coloured gas liberated in test tube A
b. a brown coloured gas liberated in test tube B
c. acolourless gas liberated in test tube A
d. acolourless gas liberated in test tube B
26. A balanced chemical equation is in accordance with-
a.Avogadro’s law
b.law of multiple proportion
c.law of conservation of mass
d.law of gaseous volumes
27. White silver chloride in sunlight turns to-
a.grey
b. yellow
c.remain white
d.red
12. A redox reaction is one in which-
a. both the substance are reduced
b. both the substance are oxidised
c. an acid is neutralised by the base
d. one substance is oxidised while the other isreduced
19. Black and white photography uses-
a. decomposition of silver chloride
b. decomposition of silver bromide
c. both
d. none of these
CLASS 10TH
SUBJECT: SCIENCE
CHAPTER 1: CHEMOCAL REACTIONS AND EQUATIONS (VSAQ)
Write the equation from 1 to 10
1) Barium chloride + Aluminium sulphate —-> Barium sulphate + Aluminium chloride
A. 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4
2) Solutions of Barium chloride and Sodium sulphate in water react to give insoluble
Barium sulphate and solution of Sodium chloride.
A. BaCl2 + Na2SO4 → BaSO4 + 2NaCl
3) Sodium hydroxide solution in water reacts with hydrochloric acid solution to produce
Sodium chloride solution and water.
A. NaOH + HCl → NaCl + H2O.
4) Potassium metal reacts with water to give potassium hydroxide and Hydrogen gas.
A. 2K + 2H2O → 2KOH + H2
5) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
A. 2H2S + 3O2 → 2H2O + 2SO2
6) Calcium hydroxide + Carbon dioxide —-> Calcium carbonate + Water
A. 2Ca(OH)2 + 2CO2 → 2CaCO3 + 2H2O
7) Barium chloride + Potassium sulphate —-> Barium sulphate + Potassium chloride
A. BaCl2 + K2SO4 → BaSO4 + 2KCl
8) Aluminium + Copper chloride —-> Aluminium chloride + Copper
A. 2Al + 3CuCl3 → 2AlCl3 + 3Cu
9) Calcium hydroxide + Carbon dioxide —-> Calcium carbonate + Water
A. 2Ca(OH)2 + 2CO2 → 2CaCO3 + 2H2O
10) In the refining of Silver, the recovery of silver from Silver nitrate solution involves
displacement reaction by Copper metal.
A. Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
11) Fe2O3 + 2Al → Al2O3 + 2Fe. Name the kind of reaction.
A. Displacement reaction
12) Fe2(SO4)3 + KOH → K2SO4 + Fe(OH)3
13) What happens when dilute hydrochloric acid is added to iron fillings?
A. Hydrogen gas and Iron chloride are produced. 2HCl + Fe → FeCl2 + H2
Balance the equation from 13 to 20
14) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
A. 2HNO3 + 2Ca(OH)2 → 2Ca(NO3)2 + 2H2O
15) BaCl2 + H2SO4 → BaSO4 + HCl
A. BaCl2 + H2SO4 → BaSO4 + 2HCl
16) SiCl4 + H2O → H4SiO4 + HCl
A.SiCl4 + H2O → H4SiO4 + 4HCl
17) Na2CO3 + HCl → NaCl + H2O + CO2
A.Na2CO3 + HCl → 2NaCl + H2O + CO2
18) Fe2(SO4)3 + KOH → K2SO4 + Fe(OH)3
A. Fe2(SO4)3 + KOH → K2SO4 + Fe(OH)3
19) Ca(PO4)2 + SiO2 → P4O10 + CaSiO3
A. Ca3(PO4)2 + SiO2 → P4O10 + 3CaSiO3
20) Al2(SO4)3 + Ca(OH)2 → Al(OH)3 + CaSO4
A. Al2(SO4)3 + Ca(OH)2 → 2Al(OH)3 + CaSO4
21) Al2(SO4)3 + Ca(OH)2 → Al(OH)3 + CaSO4
A.Al2(SO4)3 + 3Ca(OH)2 → 2Al(OH)3 + 3CaSO4
22) Explain why a ray of light passing through the centre of curvature of concave minor
gets reflected along the same path.
A. The light falling on the centre of curvature is reflected back along the same path
becauseincident ray falls on the mirror along the normal to the reflecting surface.
23) What is hyperacidity?
A. Hyperacidity is caused by excess of hydrochloric acid in stomach.
24) How antacid works to cure it?
A.Antacid is basic in nature. It neutralizes excess of acid and gives relief from pain
caused by hyperacidity.
25) Name the stimulus involved in each of the following phenomenon:
i. Phototropism
ii. Geotropism
A. (i)Light (ii) Gravity
26) How will you test for the gas which is liberated when hydrochloric acid reacts with an
active metal?
A. Bring a burning matchstick near the gas. It burns with ‘pop’ sound showing that it is
hydrogen gas.
27) Explain what is galvanisation. What purpose is served by it?
A.The process of coating the iron objects by zinc metal is called galvanisation.
Galvanisation is used to prevent rusting of iron.
28) A Mendelian experiment consisted of breeding pea plants bearing violet flowers with
pea plants bearing white flowers. What will be the result in F1 progeny ?
A. Violet flowers
29) How will you test for the gas which is liberated when hydrochloric acid reacts with an
active metal?
A. Bring a burning matchstick near the gas. It burns with ‘pop’ sound showing that it is
hydrogen gas.
30) N2 + 2H2 2NH3. Name the type of reaction.
A. It is a combination reaction.
31) Which substance is flushed in bags of potato chips to prevent rancidity?
Ans: Nitrogen
32) What is the name of the reaction in which an atom, molecule or ion loses oxygen ?
Ans: Oxidation Reaction
33) What is the chemical name of rust?
Ans: Reduction Reaction
34) What is the chemical reaction is rust ?
Ans: Iron Oxide
35) Which substance does gets oxidised in rancidity of food?
Ans: Fats present in the food
36) In which reaction generally heat is evolved ?
Ans: Combination Reaction
37) In which reaction generally heat is absorbed ?
Ans: Decomposition Reaction
38) Which reaction is used in black and white photography ?
Ans: Thermal Reaction
39) Which chemical substance is used to print impression of black and white photography
paper ?
Ans: Silver Chloride or Silver Bromide
40) What is the chemical name of quick lime ?
Ans: Calcium Oxide
41) What is the chemical name of limestone ?
Ans: Calcium Carbonate
42) What is the chemical name of slaked lime ?
Ans: Calcium Hydroxide
43) Which chemical substance is used in white washing ?
Ans: Calcium Hydroxide (Slaked Lime)
44) What is a chemical equation ?
Ans: Symbolic representation of chemical reaction is called chemical equation.
45) What is rancidity?
Ans: Spoilage of food because of oxidation of fats present in the food.
46) What is corrosion of metals ?
Ans: Degradation of metals because of formation of a layer of respective oxides,
sulphides etc. over them is called corrosion of metals.
47) Why silver metal appears black if kept in open for some days ?
Ans: Silver metal appears black because of formation of a layer of sulphide over them if
kept open for some days.
48) Why is it advisable to show the physical state of substance when writing a chemical
reaction ?
Ans: It makes a chemical reaction more informative.
49) Why displacement reaction is also known as single displacement reaction ?
Ans: In displacement reaction only one atom or ion is displaced, that’s why it is known
as single displacement reaction.
50) What is endothermic reaction ?
Ans: Chemical reactions in which heat is absorbed are called endothermic reactions. For
example: Decomposition of calcium carbonate.
51) What is exothermic reaction ?
Ans: Chemical reactions in which heat is evolved are called exothermic reactions. For
example: Reaction of calcium oxide with water.
52) Write a chemical equation when magnesium metal reacts with aqueous hydrochloric
acid to produce a solution of magnesium chloride and hydrogen gas.
A. g + 2HCL -> MgCl2 + H2
Fill in the blanks:-
1.Addition of hydrogen in a substance in a reaction is known as___________
reaction.
2.In a ________ reaction two or more substances combine to form a new single
substance.
3.Unbalanced reactions are also known as _________
4.Reactions in which heat is given out along with the products are called _________-
reactions.
5.Reactions in which energy is absorbed are known as _________ reactions.
6.When as element displaces another element from its compound, a ________
reaction occurs.
7.Those reactions, in which two compounds react by an exchange of ions to form two
new
compounds, are called ________ reactions
8.Precipitation reactions produce __________ salts.
9.Reduction is the _________ of oxygen or gain of hydrogen.
10.The digestion of food in the body is an example of ____________ reaction.
11.The addition of oxygen to a substance is called __________.
12.When calcium carbonate is heated, it decomposes to give___________ and
_________.
Solutions
1.reductioN 2.Combination 3.skeletal
4.Exothermic 5.Endothermic 6.Displacement
7.Double displacement 8.Insoluble 9.Loss
10.Decomposition reaction 11.Oxidation 12.CaO(s) and
CO2 (g)
True/ False:-
1.The number of atoms of each element is conserved in any chemical reaction.
2.Oxidation is the loss of electrons from a substance.
3.Reduction is the gain of electrons by a substance.
4.A complete chemical equation represents the reactants, products and their physical
states symbolically.
5.A magnesium ribbon burns with a dazzling flame in air (oxygen) and changes into a
white substance, magnesium oxide.
6.Rusting is a double decomposition reaction.
7.The reaction between nitrogen and hydrogen to give ammonia is an example of a
combination reaction.
8.Action of heat on ferrous sulphate is an example of decomposition reaction.
9.The formation of Na+ and CI- ions from sodium and chlorine is an example of a
redox reaction.
Solutions
TRUE
1. TRUE 2. TRUE 3. TRUE
4. TRUE 5. TRUE 6. FALSE
7. TRUE 8. TRUE 9. TRUE
CLASS: 10TH
SUBJECT: SCIENCE
CHAPTER 1: CHEMICAL REACTIONS AND EQUATIONS (SAQ)
1) “We need to balance a skeletal chemical equation.” Give reason to justify the
statement.
A. Skeltal chemical equation are unbalanced.
We need to balance chemical equation because of law of conservation of mass.
It states that ‘matter can neither be created nor be destroyed.’
Therefore chemical equation must be balanced in each and every chemical reaction.
2) Mention the chemical equation for the process of photosynthesis. When do desert
plants take up carbon dioxide and perform photosynthesis ?
A. 6CO2 + 6H2O + (energy) → C6H12O6 + 6O2
In desert plants the stomata are open at night. They take CO2 at night and is store in
the form of acid and is used during day time for photosynthesis.
3) What happens chemically when quick lime is added to water?
A. When quick lime is added to water calcium hydroxide (slaked lime) is formed with
a hissing sound and lot of heat is evolved during the reaction. The reaction involved is
CaO (s) + H2O(Quick Lime) Ca (OH) 2 (s) + Heat (Shaked Lime)
4) Why are food particle preferably packed in aluminum foil?
A. Aluminium foil do not corrode in atmosphere even if kept for a long time because
a protective coating of aluminium oxide (Al2O3) is formed on the surface of the foil
and stops any further reaction of the metal with air and water thus food particles do
not get spoiled.
5) What do you mean by a precipitation reaction? Explain by giving examples.
A. A chemical reaction in which an insoluble substance (precipitate) is formed is
called precipitation reaction. For example,
AgNO3 + NaCl AgCl + NaNO3
6) Give an example of a double displacement reaction .
A. Reaction Between silver nitrate (AgNO3) and Sodium chloride (NaCl) is an example
of double displacement reaction. Here Silver donates its ion to Sodium chloride ion
and pick up nitare ion to form silver chloride and sodium Nitrate.
AgNO3 + NaCl → AgCl + NaNO3
7) Define oxidation with ample examples
A. In a chemical reaction, when the oxygen is added to the element to form its
respective oxide it is the element being oxidised.
4Na(s) + O2(g) → 2Na2O(s)
H2S + O2 → H2O + SO2
8) Explain rancidity.
A. The condition produced by the aerial oxidation of the oil and fat present in the
food material that produces an unpleasant taste and smell.
The rancidity is retarded when the food is kept inside the refrigerator since the low
temperature does not promote the oxidation reaction.
9) Oil and fat containing food items are flushed with nitrogen. Why?
A. Oil and fat containing items get rancid due to oxidation with atmospheric oxygen.
To prevent rancidity food items are flushed with nitrogen. Nitrogen do not reacts
with oil and fat containing items.
10) Why should a magnesium ribbon be cleaned before burning in air?
A. Magnesium rubber should be cleaned before burning in air because
Magnesium metal reacts with the atmospheric oxygen and forms Magnesium Oxide
(MgO) layer which is a very stable compound.
In order to prevent further reactions with Oxygen, it is therefore necessary to clean
the ribbon by to remove the layer of MgO.
11) Explain rancidity.
A. The condition produced by the aerial oxidation of the oil and fat present in the
food material that produces an unpleasant taste and smell.
The rancidity is retarded when the food is kept inside the refrigerator since the low
temperature does not promote the oxidation reaction.
12) What is reduction?
A. In a chemical reaction, when the oxygen is being removed from the compound
then it is said to be reduced.Example:
CuO(s) + H2(g) → Cu(s) + H2O(l)
2HgO → 2Hg + O2
13) Whatdo you mean by the skeletal chemical equation ?
A. A chemical reaction written in the form of symbols and formulae is called skeletal
chemical equation. The above word-equation can be written as a balanced chemical
equation:2Mg + O2 → 2MgO
In this equation there are two oxygen atom on the LHS side while there is only one
oxygen atom on the RHS side.
14) What is meant by the combination reaction?
A. The reaction in which two or more element or compound combine together to
form a single compound are called Combination reaction.Example:
CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat
C(s) + O2(g) → CO2(g)
15) Why is respiration considered an exothermic reaction?
A. Respiration is considered as an exothermic reaction because in respiration
oxidation of glucose takes place which produces large amount of heat energy.
C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + Energy
16) What do you mean by a precipitation reaction?
A. A reaction in which an insoluble solid (called precipitate) is formed is called a
precipitation reaction.For Example:
Na2CO3 (aq) + CaCl2 (aq) → CaCO3 (s) + 2NaCl (aq)
In this reaction, CaCO3is obtained as a precipitate. Hence, it is a precipitation
reaction.
sunlight
17) What is the important function of presence of ozone in earth’s atmosphere ?
A. Ozoneshields our planet from the UV radiations, as these ultraviolet rays are
harmful for all the living things.
herefore, any change in the size of this layer will automatically affect the life of
people, plants and animals living on earth.
18) A student prepares aqueous solutions of the following salts : Copper sulphate,
ferrous sulphate , Sodium sulphate, Barium chloride. Write the colour of weach
solution formed.
A. Copper sulphate – Green
ferrous sulphate – Pale green
Sodium sulphate – Milky white
Barium chloride -- White
19) “We need to balance skeletal chemical equation.” Give reasons to justify the
statement.
Ans: Skeletal chemical equations are unbalanced. We need to balance chemical
equation because of law of conservation of mass. It states that ‘matter can neither be
created nor be destroyed’. Therefore chemical equation must be balanced in each
and every chemical reaction.
20)
a. Write a balanced chemical equation for process of photosynthesis.
b. When do desert plants take up carbon dioxide and perform photosynthesis ?
Ans: (a) 6C (g) + 6 O (l) + 6 (g)
(b) In desert plants stomata are open at night. They take C at night and is
stored in the form of acid and is used during day time for photosynthesis.
21) Write the chemical equation of the reaction in which the following changes have
taken place with the example of each:
a. Change in colour
b. Change in temperature
c. Formation of precipitate
Ans:(a) Cu (s) + 2AgNO3 (aq)———–> Cu(NO3)2(aq) + 2Ag
The solution will become blue in colour and shiny silver metal will be deposited.
(b) NaOH + HCl ———–> NaCl + H2O+ heat
The temperature will increase because heat is evolved.
(a)Pb(NO3)2 (aq) + 2KI (aq)———–> Pbl2 (s) + 2KNO3 (aq)
Yellow precipitate of Pb will be formed.
22) How can you help your mother in keeping the fried items so that they do not
develop a bad shell and their shelf life is increased ?
Ans: Since we cannot pack the fried items in the atmosphere of nitrogen at home but we
can increase their shell life by keeping them in airtight containers. In this way they do
not come in contact with oxygen. Their shell life can further be increased by keeping
them in fridge at low temperature.
23) Write any two limitations of chemical equations.
Ans: A chemical equation does not provide the following informations :- 1)Whether
the reaction is fast, slow or instantaneous i.e. the rate of reaction is not shown by a
chemical equation.
2)Whether the reaction goes to completion or is stopped in between i.e. the extent to
which a reaction takes place is not known from a chemical equation.
24) Distinguish between an exothermic and an endothermic reaction. Amongst the
following reactions, identify the exothermic reaction and the endothermic reaction:
1. Heating coal in the air to form carbon dioxide.
2. Heating limestone in lime kiln to form quick lime.
Ans: Exothermic reactions are those reactions in which heat is evolved. Endothermic
reactions are those reactions in which heat is absorbed.
1. Heating coal in the air to form carbon dioxide is exothermic reaction.
2. Heating limestone in lime kiln to form quick lime is an endothermic reaction.
25) Why does the blue colour of copper sulphate solution changes to green colour
when an iron nail is dipped? Write a chemical equation.
Ans: The blue colour of copper sulphate solution changes to green colour because
copper from copper sulphate is replaced by iron and forms ferrous sulphate which is
green coloured.
Fe(s) + CuS -> FeS (aq) + Cu
26) State any two observations in an activity which may suggest that a chemical
reaction has taken place. Give examples in support of your answer.
Ans: 1) In a tube take a small amount of solid sodium carbonate N . To this add
a few drops of hydrochloric acid.
2) a colourless and odourless gas will evolve which shows that a chemical reaction
has taken place. The gas will turn lime water milky when passed through it.
27) A housewife wanted her house to be white washed. She brought 10 kg of quick
lime from the market and dissolved in 30 litres of water. On adding lime to water,
she noticed that the water started boiling even when it was not heated. Give
reasons for her observation. Write the corresponding equation and write the name
the product formed.
Ans: A suspension of slaked lime also called calcium hydroxide is formed when water
is added to quick lime.
CaO (s) + O (l) Ca(O (s) + heat
(slaked lime)
Since the reaction is highly exothermic, the solution started although it was not being
heated. The suspension of slaked lime is allowed to cool for some time, preferably
overnight. It is then decanted and the liquid obtained is used for white washing.
28) What is colour of ferrous sulphate crystals? How does this colour changes after
heating?
Ans: The colour of ferrous sulphate is green. The colour changes to reddish brown on
heating due to formation of iron(III) oxide.
29) Why oes the colour of copper sulphate solution changes when an iron nail is dipped
in it? Write two observation.
heat
Ans: It is because displacement reaction takes place. Iron displaces copper from copper
sulphate solution and forms pale green coloured solution of FeS and reddish brown
copper metal gets deposited.
Fe(s) + CuS (aq) FeS (aq) + Cu(s)
30) Why are decomposition reactions called opposite of combination reactions? Write
equations for these reactions.
Ans: In decomposition reaction, a compound is broken down into simpler compounds or
elements, eg.
CuC (s) CuO(s) + C (g)
Combination reaction is a reaction in which two or more elements or compounds
combine to form a new compound, eg.
(g) + 3 (g) 2N (g)
Thus decomposition and combination reaction are opposite to each other.
31) Why should a magnesium ribbon be cleaned before burning in air?
Ans: Magnesium is very reactive metal. When stored it reacts with oxygen to form a
layer magnesium oxide on its surface. This layer of magnesium oxide is quite stable and
prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned
by sand paper to remove this layer so that the underlying metal can be exposed into air.
32) Why is the amount of gas collected in one of the test tubes in the activity of
‘electrolysis of water’ double of the amount collected in the other? Name this gas.
Ans: Water( O) contains two parts of hydrogen and one part of oxygen. Therefore, the
amount of hydrogen and oxygen produced during electrolysis of water is in a 2:1 ratio.
During electrolysis, since hydrogen goes to one test tube and oxygen goes to another,
the amount of gas collected in one of the test tubes is double of the amount collected in
the other.
33) Why is respiration considered an exothermic reaction? Explain.
heat
Ans: energy is required to support life. Energy in our body is obtained from the food we
eat. During digestion, large molecules of food are broken down into simpler substances
such as glucose. Glucose combines with oxygen in the cells and provides energy. The
special name of this combustion reaction is respiration. Since energy is released in the
whole process, it is an exothermic process.
(aq) + 6 (g) 6 (g) + 6 + energy
Oxygen Carbon Water
Dioxide
34) A shiny brown-coloured element ‘X’ on heating in air becomes black in colour.
Name the element ‘X’ and the black coloured compound formed.
Ans: ‘X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO).
The equation of the reaction involved on heating copper is given below:
2Cu + 2CuO
(shiny brown in colour) (Black in colour)
35) Why do we apply paint on iron articles?
Ans: Iron articles are painted because it prevents them from rusting. When painted, the
contact of iron articles from moisture and air is cut off. Hence, rusting is prevented their
presence is essential for rusting to take place.
CLASS: 10TH
SUBJECT: SCIENCE
CHAPTER 1: CHEMICAL REACTIONS AND EQUATIONS (LAQ)
1) What is a balanced chemical equation? Why should a chemical equation be balanced?
A. A balanced equation is the one in which number of different atoms on both the
reactant and product sides are equal.
Balancing chemical equation is necessary for the reaction should obey The Law of
Conservation of energy.
Balancing the chemical equation has no defined method and is purely a trial and
error attempt.
2) Why is respiration considered to be an exothermic reaction?
A. For the survival of life, we require energy.
We obtain this energy from the food we eat. The food molecules, through the
process of digestion, is broken down into a simpler molecule like glucose.
These substances come in contact with the Oxygen present in our body cells to
form Carbon dioxide and water along with a certain amount of energy
(Respiration process).
Since the energy is in the form of heat (that maintains our body temperature) the
respiration is considered to be an exothermic reaction.
The reaction taking place is:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
3) Why are decomposition reactions called the opposite of Combination reactions? Write
equations for decomposition reactions.
A. Combination reaction is said to be the reaction between two or more molecules to
form a larger molecule; whereas the decomposition reaction is defined as the splitting of
larger molecules into two or more smaller molecules.
This essentially explains that the decomposition reaction is the opposite of the
combination reaction.
the decomposition reaction is endothermic since heat from the surrounding or induced
heat is used to break the bonds of the larger molecule.
Few examples of decomposition reactions are:
ZnCO3 → ZnO + CO2
CaCO3 + Energy → CaO + CO2
2HgO → 2Hg + O2
4) What is the difference between displacement and double displacement reactions?
Write relevant equations for the above.
A. A displacement reaction is the one when a more reactive substance displaces a less
reactive one from its salt solution whereas a double displacement reaction is the one
where a mutual exchange of ions happens between two compounds.
In a displacement reaction, only a single displacement takes place whereas in the double
displacement reaction, as the name suggests two displacement takes place between the
molecules.
Example:
Displacement reaction:
Mg + 2HCl → MgCl2 + H2
Double displacement reaction:
2KBr + BaI2 → 2KI + BaBr2
5) What do you mean by a precipitation reaction? Explain by giving examples.
A. When two solutions containing soluble salts are combined, a double displacement
reaction takes place in which the ions are exchanged between the compounds.
When one of such compounds formed is in solid form (that is insoluble in aqua) then it
settles down at the bottom of the container.
This solid is known as the precipitate and the respective reaction is termed as the
precipitation reaction.
Few examples of precipitation reactions are:
CdSO4(aq) + K2S(aq) → CdS(s) + K2SO4(aq)
2NaOH(aq) + MgCl2(aq) → 2NaCl(aq) + Mg(OH)2(s)
6) Oil and fat containing food items are flushed with nitrogen. Why?
A.Nitrogen is an inert gas (at room temperature) and does not easily react with oil and
fat containing items. On the other hand, oxygen reacts with food substances and makes
them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to
remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of
oil and fat containing food items is avoided.
7) Write one equation each for decomposition reactions in which energy is supplied in
the form of heat, light or electricity.
A.Thermal decomposition reaction (Thermolysis)
Decomposition of potassium chlorate: When heated strongly, potassium chlorate
decomposes into potassium chloride and oxygen. This reaction is used for the
preparation of oxygen. 2KClO3 + Heat → 2KCl + 3O2
Electrolytic decomposition reaction (Electrolysis)
Decomposition of sodium chloride: On passing electricity through molten sodium
chloride, it decomposes into sodium and chlorine.
2NaCl 2Na + Cl2
Photodecomposition reaction (Photolysis)
Decomposition of hydrogen peroxide: In the presence of light, hydrogen peroxide
decomposes into water and oxygen.
2H2O2 2H2O
8) What Is Meant By: (I) Precipitation Reaction (Ii) Exothermic Reaction (Iii) Oxidation
Reaction. Write Balanced Chemical Equation For An Example Of Each
i. The reaction in which two compound exchange their ions and the product formed
is insoluble in water is called precipitation reaction.
Example: CuSO4(aq) +H2→Cu+H2So4
ii. The reaction in which heat is evolved.
Example: C6H12O6 + 6O2 → 6Co2 + 6H2O + heat
iii. The reaction in which O2 is added or H2 is removed or loss of electron take place.
Example: 2Cu + O2 → 2CuO
9) Name The Substance Oxidised And Reduced, And Also Identify The Oxidising Agents
And Reducing Agents In The Following Reactions: (I) Fe2O3 + 3Co → 2Fe + 3CO2. (Ii)
3MnO2 + 4Al → 3Mn + 2Al2O3. (Iii) H2S + SO2 → S + H2O
i. Fe2O3+3CO→ 2Fe + 3CO2
Fe2O3 is reduced because oxygen has been reduced and CO is oxidised because O
has been added to it. Fe2O3 has acted as oxidising agent and CO has acted as
reducing agent.
ii. 3Mno2 +4Al → 3Mn + 2Al2O3
MnO2 is reduced because O has been removed from it to give Mn. Al has been
oxidised because O has been added to it to obtain Al2O3. MnO2 is the oxidising
agent while Al is reducing agent.
iii. 2 H2S + SO2→3S +2H2O
H2S has lost H to change into S. Thus, it has been oxidised. SO2 has been lost O to
change into S. Thus, it has been reduced. H2S has acted as reducing agent while
SO2 has acted as oxidising agent.
10) (A) Can We Stir Silver Nitrate Solution With A Copper Spoon? Why Or Why Not?
Support You Answer.
(B) Why A Brown Coating Is Formed On The Iron Rod When Iron Rod Is Kept Dipped In
Copper Sulphate Solution For Some Time? What Change Will Be Observed In The
Colour Of The Solution.
(C) A Green Coating Develops On The Copper Vessel In The Rainy Season. Why?
A. We cannot stir silver nitrate with a copper spoon. Copper being more reactive than
silver, displaces silver from silver nitrate, that is, a reaction will take place.
B. Iron being more reactive that copper, displaces copper from copper sulphate by the
following reaction:Fe + CuSo4→ FeSo4 + Cu
Brown coating of copper is deposited on the iron rod. Colour of the solution
becomes green due to the formation of ferrous sulphate.
C. Copper reacts with moist carbon dioxide in the air in rainy season and a green
coating is formed. This is due to
D. the formation of copper carbonate.
11)(A) Explain any two ways by which food industries prevent rancidity ?
(B) Discuss the importance of decomposition reaction in metal industry with three
points.
(a) (i) rancidity can be prevented by adding antioxidants to food containing fats and oils.
Eg.ButylatedHydroxy Anisole is added to butter as an antioxidant.
(ii) it can be prevented by packing fat and oil containing food s in nitrogen gas.
(b)(i) Molten NaCl is electrolytically decomposed to form sodium metal.
(ii) aluminium metal is obtained by electric decomposition of bauxite ore mixed with
cryolite.
(iii) carbonate ores are thermally decomposed to give metal oxide which on reducing give
metal.
11) Describe an activity to observe what happens when quick lime is added to water
taken in a beaker. State two important observations and name the type of reaction
taking place.
ANS: Aim: To observe what happens when quicklime is added to water taken in a beaker.
Material Required:Quicklime(calcium oxide), water, beaker
Procedure:
1)Take 5g of calcium oxide in a beaker.
2)Add water to it slowly.
3)Touch the beaker.
4)Note down the observation.
Observation: Calcium oxide reacts with water vigorously to form calcium hydroxide with
the evolution of heat.
Chemical Reaction:
CaO(s) + O(l) Ca (aq) + heat
Calcium Oxide water slaked lime
Conclusion: The reaction between calcium oxide and water is a combination reaction. It is
an exothermic process because heat is evolved.
12) Describe an activity to demonstrate the change that takes place when white silver
chloride is kept in sunlight. State the type of chemical reaction which takes place.
Ans:
Aim: To demonstrate the change that takes place when white silver chloride is kept in
sunlight.
sunlight
Materials Required:AgN (aq), NaCl(aq), test tubes.
Procedure:
1) Take 5ml of silver nitrate solution in a test tube.
2) Prepare sodium chloride solution in another test tube.
3) Add sodium chloride solution into test tube containing sodium nitrate
solution.
4) Observe the colour of silver chloride to grey silver metal. Dry it with the
help of filter papers and place it on the watch glass.
5) Place the watch glass under sunlight for some time.
6) Observe the colour of the silver chloride after some time.
Observation: White silver chloride turns grey in sunlight because silver metal is formed.
Chemical Reaction:
2AgCl(s) 2Ag(s) + C (g)
Explanation: Silver Chloride is photosensitive. It decomposes in the presence of sunlight to
form silver metal and chlorine gas.
Conclusion: Decomposition of silver chloride in presence of sunlight is photochemical
decomposition reaction.
13) Distinguish between a displacement reaction and a double displacement reaction.
Identify the displacement and double displacement reaction from the following
reactions.
(i)HCl (aq) + NaOH(aq) NaCl (aq) + O(l)
(ii)Fe(s) + CuS (aq) FeS (aq) + Cu(s)
Ans: Displacement reaction is a reaction in which more reactive metal can displace less
reactive metal from its salt solution.
Double displacement reaction are those reactions in which compounds exchange their ions
to form two new compounds.
(i)Double displacement reaction
(ii)Displacement reaction
14) Explain the following terms with one example each
(a) Corrosion
(b) Rancidity
Ans: (a) Corrosion:
Corrosion is defined as a process where materials, usually metals, deteriorate as a
result of a chemical reaction with air, moisture, chemicals etc.
For example, iron, in the presence of moisture, reacts with oxygen to form hydrated
iron oxide.
4Fe + 3 + n O 2F .n O
Hydrated iron oxide
This hydrated iron oxide is rust.
(b) Rancidity:
The process of oxidation of fats and oils that can be easily noticed by the change in
taste and smell is known as rancidity.
For example, the taste and smell of butter changes when kept for long.
Rancidity can be avoided by:
1) Storing food in air tight containers
2) Storing food in refrigerator
3) Adding antioxidants
4) Storing food in an environment of nitrogen