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Revision Question Bank
Chemical Reactions and Equations
1.
Give the reaction taking place during respiration in the body cells.
Sol:
The reaction taking place during respiration is as follows
C6H12O6  aq   6 O2  aq  

Glucose
Oxygen
6 CO2  aq  6H2O  l  Energy
Carbon
dioxide
2.
water
Name the reaction in which one of the reacting species loses, while the other gains
electrons.
Sol:
Redox reaction is that reaction, in which one of the reacting species loses, while the
other gains electrons.
3.
Why is decomposition reaction called opposite of combination reaction? Write
equations for these reactions.
Sol:
In a combination reaction, two or more substances
combine together to form a single new substance whereas, in a decomposition reaction,
a single substance is broken into two or more new substances, e.g.
Combination reaction
Burn
2H2  g  O2  g  
2H2 O  g 
Hydrogen
Oxygen
water
Decomposition reaction
Electricity
2H2 O  l  
 2H2 O  g 
water
Hydrogen Oxygen
It is clear that decomposition reaction is the reverse of combination reaction.
4.
Write balanced chemical equation for each of the following.
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(a) Methane burns in air to give carbon dioxide, water and heat.
(b) Ferric oxide reacts with sulphuric acid to give ferric sulphate and water.
Sol:
(a) CH4  g  2O2  g  
 CO2  g   2H2O  g   Heat
Methane
Oxygen
Carbon dioxide
water
(b) Fe2O3  s  3H2SO4  aq  
 Fe2 SO4 3
Iron(III)
oxide
5.
Sulphuric
acid
aq   3H2O  l 
Iron(III)
sulphate hydrate
water
All single displacement reactions are irreversible reactions. Explain.
Sol:
Single displacement reactions involve the displacement of a less reactive element by a
more reactive element and the less reactive element precipitates out. But the reverse of
this is not possible because the less reactive element cannot
displace back the more reactive element.
For example,
Fe  s  CuSO4  aq  
 FeSO4  aq  Cu 
Iron
6.
Blue
Green
copper
Write equations for each decomposition reactions where energy is supplied in the form
of heat, light or electricity.
Sol:
(i) The decomposition reaction, when energy is supplied in the form of heat,
Heat
CaCO3  s  
CaO3  CO2  g 
Calcium
carbonate
calcium
oxide
carbon
dioxide
(ii) The decomposition reaction, when energy is supplied in the form of electricity
Electricity
2H2O  l  
2H2  g  O2  g 
water
Hydrogen
Oxygen
(iii) The decomposition reaction, when energy is supplied in the form of sun-light.
sunlight
2AgBr  s  
2Ag  Br2 
silver bromide
7.
silver
bromine
In what way are chemical equations useful for us?
Sol:
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A chemical equation is useful for us because it gives us the following informations
(i) Names of the reactants, which take part in the chemical reaction,
(ii) Names of the products formed,
(iii) Number of moles of each reactant and product,
(iv) Mass of each substance involved in the reaction.
(v) Physical state of all reactants and products,
(vi) Physical conditions of a reaction.
8.
Lime water turns milky when carbon dioxide gas is passed through it and further turns
colourless when excess of carbon dioxide is bubbled through it. Explain.
Sol:
Lime water is Ca (OH)2 solution. When CO2 is passed through lime water, it turns milky
due to the formation of insoluble CaCO3 precipitate. But when excess of CO2 is passed
through the solution, calcium bicarbonate is formed, which is soluble in
water and so the solution again turns colourless.
Ca OH 2  CO2 

Calcium
hydroxide
CaCO
carbon
dioxide
3
Calcium carbonate
CaCO
3
calcium
carbonate
 Milky suspension 
 H2O
water
 H2O CO2 
 Ca  HCO3 2  aq 
water
carbon
dioxide
 Excess 
Soluble
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Chemical Reaction {Chapter Test}
M:M: 30
1.
M: Time: 40Mins.
Why do iron railings on roadside get rusted if not painted?
[1]
Solution: If it is not painted, the iron surface will remain in contact with air and
moisture. So, oxide layer called the rust, will deposit on its surface.
2.
When the powder of a common metal is heated in an open China dish, its colour turns
black. However, when hydrogen is passed over the hot black substance so formed, it
regains its original colour.
Based on the given information, answer the following questions
(a) Name the metal initially taken in the powder form.
(b) What type of chemical reaction takes place in each of the two given steps? Write
balanced chemical equations for both reactions.
[2]
Solution: (a) Copper metal is taken initially in the powder form.
(b) In step I combination reaction/oxidation reaction takes place.
Heat
2Cu  s   O2  g  
 2CuO  s 
Oxidation
Copper (brown)
Oxygen
Copper oxide (Black)
In step n, reduction reaction takes place.
Reduction
CuO  s   H2  g  
 Cu  s 
Copper oxide
3.
Hydrogen
 H2O  g 
Copper (Brown)
Water
Draw the apparatus for electrolysis of water. Why is the amount of hydrogen produced,
double the amount of oxygen?
[3]
Solution: The apparatus for electrolysis of water is shown below
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Number of molecules of hydrogen produced are double the molecules of oxygen
produced, according to the reaction.
Electrolysis
2H2O  l  
 2H2  g   O2  g 
water
4.
Hydrogen
Oxygen
A piece of sodium is dropped in water. What happens in this process? Which gas is
evolved in the process? How is it detected?
[4]
Solution: When sodium is dropped in water, it reacts violently with water.
During this exothermic reaction, hydrogen gas is evolved.
To detect this gas, it is passed through soap solution. The bubbles formed rise up and
burn with a popping sound if a burning matchstick is brought near it.
5.
Balance the following chemical equations
[3]
(a) HNO3 + Ca(OH)2 
 Ca(NO3)2 + H2O

 Fe2O3 + SO2 + SO3
(b) FeSO4 
 CO2 + H2O + Heat
(c) C6H12O6 + O2 
Ca  NO3 2  2H2O
Solution: (a) 2HNO3  Ca OH 2 
Nitric acid
(b)
Calcium hydroxide
Calciumnitrate
Water

2FeSO4 
 Fe2O3  SO2  SO3
Iron(II)sulphate
Iron(III)Oxide
Sulphur
dioxide
Sulphur
tioxide
(c) C6H12O6  6O2 
 6CO2  6H2O Heat
Glucose
6.
Oxygen
Carbondioxide
Water
A solution of sodium sulphate is added to barium chloride solution. Write a balanced
chemical equation for the reaction and the type of reaction.
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Solution: Na2SO4  aq   BaCl2  aq  
 BaSO4  s  2NaCl
sodium
sulphate
Barium
chloride
Barium
suphate
sodium
chloride
It is a double displacement reaction because in this reaction, two different ions in the
reactant molecules are displaced by each other.
7.
Write balanced equations for the following, mentioning the type of reaction involved.
(a) Aluminium + Bromine 
 Aluminium bromide
(b) Calcium carbonate 
 Calcium oxide +Carbon dioxide
(c) Silver chloride 
 Silver + Chlorine
[3]
Solution: (a) 2Al(s) + 3Br2 (l) 
 2AlBr3 (s)
It is combination reaction because in this reaction, two reactants, Aland Br2 react
together to form a single product (AlBr3)

(b) CaCO3 (s) 
 CaO(S)+CO2(g)
It is thermal decomposition reaction because this reaction involves the use of energy in
the form of heat for decomposition of the reactant (CaCO3).
Sunlight
(c) 2AgCl(s) 
 2Ag(S)+Cl2(g)
It is photochemical decomposition or photolysis because this reaction involves the use of
light energy for the purpose of decomposition of the reactant (AgCl).
8.
What is rancidity? How is it prevented? Why do we fill chips packets with nitrogen gas?
How does it help?
[4]
Solution: Rancidity is the change in smell and taste of oil or fat product due to oxidation.
It is prevented by using antioxidants or cutting the supply of oxygen.
Chips packets are flushed with nitrogen gas to shield the chips from oxygen supply.
Nitrogen does not cause rancidity of food. This helps in preserving the taste and smell of
food. Manufacturers and shopkeepers are helped as shelf life for the chips is prolonged.
9.
Define rusting. How does it occur? Write the reaction involved. What are its effects?
Suggest two ways of preventing rusting.
[4]
Solution:
Rusting is corrosion of iron due to the action of oxygen and moisture on its surface.
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It occurs in the presence of moist air due to the following reaction
2Fe  s   H2O O2  g  
 Fe2O3 s   H2
Iron
water
Oxygen
Iron III oxide
Hydrogen
g 
Rusting weakens iron and damages structures such as doors, bridges, cars, etc.
Rusting can be prevented by painting the iron surface or galvanising it (with zinc).
9. Why CuSO4 solution can't be stored in iron container? Give chemical equation also.
[2]
Solution: CuSO4 solution can't be stored in iron container because, copper is less
reactive than iron. The more reactive metal (iron) displaces the less reactive metal
(copper) from its solution (copper sulphate solution).
Fe  s  CuSO4  aq  
Cu  s 
Iron
Copper sulphate
 Blue
Copper
FeSO
4
Ferrous sulphate
 Light green colour 
10. Complete the reactions and state their types.
[2]

(a) 2KClO3 (s) 

(b)N2(g) + 3H2(g) 

How are the two reactions different?

Solution: (a) 2KClO3  s  
2KCl  s  3O2  g 
Potassium
chloride
(b) N2  g   3H2  g 
Potassium
chloride
Oxygen
2NH3  g 
CaO  s  H2 O  l  
Ca OH 2  aq 
calcium
chloride
water
Calcium
hydroxide
It is combination reaction.
The decomposition reaction involves breaking down of a compound into two or more
substances. In combination reaction, two or more substances combine with each other
to form one compound.
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Acids, Bases and Salts
1. How is plaster of Paris chemically different from gypsum? How can they be inter
converted? Write one use of plaster of Paris.
Sol:
Plaster of Paris is chemically different from gypsum, in terms of water of crystallisation.
Gypsum has two moles of water per mole of calcium sulphate
(CaSO4.2H2O). Plaster of Paris has half a mole of water per mole of calcium sulphate (or
two moles of calcium sulphate share one water molecule, i.e.,
1
CaSO4. H2O or 2CaSO4.H2O
2
Gypsum, on heating at around 120oC, gets converted to Plaster of Paris (POP) and POP,
upon hydration, gets converted to gypsum.
1
1
120o C
CaSO4 .2H2O 
CaSO4 . H2O 1 H2O
2
2
Gypsum
Plaster of paris
1
1
CaSO4 . H2O 1 H2O  CaSO4 .2H2O
2
2
Gypsum
POP
Use of Plaster of Paris It is used as plaster for supporting fractured bones in the right
position.
2. Given below are the pH values of four different liquids
7.0,14.0,4.0,2.0
Which of these could be that of
(a) lemon juice
(b) distilled water
(c) 1 M, sodium hydroxide (NaOH) solution (d) tomato juice
Sol:
(i) The pH value of lemon juice = 2.0
(ii) The pH value of distilled water = 7
(iii) The pH value of 1 M, sodium hydroxide (NaOH) solution =14.0
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(iv) The pH value of tomato juice = 4.0
3. State Arrhenius concept of acids and bases. Select a strong acid and a weak base from
amongst the following substances H2CO3, HNO3, NaOH, NH4OH.
Sol:
According to Arrhenius theory, acids are substances which provide H+ ions when
dissolved in water, whereas bases are substances which when dissolved in water give OH–
ions, (hydroxide ions). Among the given substances, HNO3 is a strong acid and NH4 OH is a
weak base.
i.e., Strong acid = HNO3
Weak base = NH4 OH
4. A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10
Sol:
(d) Its pH is likely to be 10 [Bases turn red litmus blue]. pH range of bases is from 8 to 14.
5. What effect does the concentration of H+(aq) have on the acidic nature of the solution?
Sol:
Acidity of a solution increases with increase in the concentration of hydronium ion (H3
O+).
6. Given two test-tubes 'A' and 'B'. Test-tube 'A' contains blue litmus solution and test-tube
'B' contains red litmus solution. A solution with pH 2 is added in both the test-tubes, in
which of the two would you observe a colour change?
Sol:
The colour of solution in test-tube 'A' will change, i.e., blue litmus solution will turn to red.
[The substance with pH 2 is acidic in nature and acids turn blue litmus red]. In test-tube
'B', no colour change is observed.
7. Write the chemical reaction involved when zinc is added to warm dilute sodium
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hydroxide solution. Can all metals react with caustic soda solution?
Sol:
When zinc is added to warm dilute sodium hydroxide solution, the following reaction
takes place
2NaOH
Sodium hydroxide
 Zn 
 Na2ZnO2  H2 
Zinc
Sodium zincale
Hydrogen
No, all metals do not react with caustic soda.
8. To an aqueous solution of sodium hydroxide, a few drops of phenolphthalein were added.
What do you observe? To this solution small amount of dilute HCl was added. What do you
observe now? Explain your answer.
Sol:
On adding phenolphthalein to sodium hydroxide solution, the solution turns pink but
when acid is added to it, the solution again turns colourless. This is because
phenolphthalein gives pink colour in alkaline medium. When acid is added to the pink
solution, the acid neutralises the base and the solution ultimately turns colourless.
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Acids , Bases and salts {Chapter Test}
M:M: 30
M: Time:
40Mins.
1.
What is the chemical name for baking soda? How is it produced? Why is it used in
baking? Write the reactions involved.
[4]
Solution:
Baking soda is sodium hydrogen carbonate (NaHCO3).
Production method It is obtained when ammonia is passed over brine solution in an
environment of CO2.
NaCl  H2O CO2  NH3 
 NH4Cl  NaHCO3
sodium
chloride
water
carbon
dixide
ammonia
ammonium
chloride
sodium
bicarbonate
On heating, it decomposes to give CO2. It is thus used for baking as it makes the cake
light and spongy.

2NaHCO3 
 Na2CO3  H2O CO2
sodium
bicarbonate
2.
sodium
carbonate
water
carbon
dioxide
(a) Out of HCl and CH3COOH which solution has higher value of pH?
(b) A student tests a sample of drinking water and reports its pH as 6 at room
temperature, which compound do you think may have been added to water?
[2]
Solution: (a) CH3COOH is a weak acid. It ionises partially and therefore it has a higher
pH value.
(b) The compound added to water may be bleaching powder. In water bleaching
powder liberates chlorine which makes its pH6 (i.e., the solution becomes
acidic).
3.
Give reasons for the following
(a) Iron develops brown layer on its surface in air.
(b) Copper develops green colour appearance in air.
(c) Iron articles are galvanised.
[3]
Solution: (a) A layer of oxide (Fe2O3) develops on iron surface due to its reaction with
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oxygen and moisture in air. This is called rusting.
(b) In air, a layer of green coloured basic copper carbonate is formed on the
surface of copper.
(c) To prevent the iron articles from corrosion (rusting), they are galvanised.
4.
What is water of crystallisation? Describe an activity to demonstrate that crystals
contain water of crystallisation.
[3]
Solution: Fixed number of water molecules present in one formula unit of a salt is
called water of crystallisation. It is responsible for the shape and colour of the crystal.
Aim To show that crystals contain water of crystallisation.
Materials Required Copper sulphate crystals, burner, boiling tube and holder.
Procedure
(i) Take a few crystals of CuSO4 and heat them in a boiling tube.
(ii) The blue colour of crystals vanishes. Small water droplets appear on the walls of
boiling tube on the upper sides.
(iii) Stop heating and add 4-5 drops of water in the test tube.
(iv) Crystals again turn blue.
Thus, hydrated copper sulphate crystals are blue in colour and have formula
CuSO4 .5H2 O
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5.
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Define chlor-alkali process. What are the uses of its products?
[3]
The process of decomposing the aqueous solution of common salt to form NaOH by
passing electricity is called chlor-alkali process.
Electric
2NaCl  aq   2H2O  l  
2NaOH  aq  Cl2  g   H2  g 
Current
sodium
hydroxide
Chlorine
 At anode
Hydrogen
 At cathode 
Chlorine is used for the treatment of water in wells and pools, in making pesticides,
CFCs and PVC manufacture.
Hydrogen is used in fuels, manufacture of ammonia and margarine.
Sodium hydroxide is used for de-greasing metals in paper making, for soap, detergents
and artificial fibres making.
6.
Write the chemical equations for the reaction of
(a) potassium
(b) calcium, with air and water.
(c) Give two examples of metals which don't react with air and water.
[3]
Solution:
(a) Potassium Reaction with air
4K+O2 
 2K2O
Reaction with water
2K+2H2O 
 2KOH+H2
(b) Calcium Reaction with air
2Ca + O2 
 2CaO
Reaction with water
 Ca(OH)2 +H2
Ca+ 2H2O 
Silver and gold do not react with water.
7.
Distinguish between strong acid and weak acid with examples.
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Solution:
8.
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Differences between strong acid and weak acid are as follows
S. No.
Strong Acid
Weak Acid
(i) Ionise almost completely in
Ionise only partially or to lesser extent in
water.
water.
(ii) Give more number H+ ions in
Give less number of H+ ions in water.
water.
(iii) pH value is less
pH value is more.
(iv) e.g., cone sulphuric
e.g., Acetic acid(CH3COOH)
acid (H2SO4).
What is aqua regia? How is it obtained? State its properties.
[2]
Solution:
Aqua regia is a reagent, which can dissolve inactive metals like gold,
platinum, etc.
Aqua regia is obtained by adding cone. HCl (3 parts) and cone. HNO3 (1 part).
Properties It has highly corrosive and fuming nature.
9.
(a) For making cake, baking powder is taken. If at home your mother uses baking soda
instead of baking powder in cake,
(i) How will it affect the taste of the cake and why?
(ii) How can baking soda be converted into baking powder?
(iii) What is the role of tartaric acid added to baking soda?
(b) Name the acid present in ant sting and give its chemical formula. Also give the
common method to get relief from the discomfort caused by the ant sting.
Solution:
[4]
(a) (i) Baking powder has tasty tartaric acid which reacts with Na2 CO3
produced during decomposition of NaHCO3 and neutralises it, so if tartaric acid is not
present, the cake will taste bitter due to the presence of sodium carbonate (Na2 CO3).
(ii) By adding tartaric acid to baking soda we can form baking powder.
(iii) Tartaric acid neutralises the sodium carbonate formed during decomposition of
NaHCO3 hence, it makes the cake tasty and not bitter in taste.
(b) The acid is formic acid (HCOOH). If baking soda (a base) is applied on the sting
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(biting area), it gives relief.
10. (a) A metal carbonate X on reacting with an acid gives a gas which when passed
through a solution Y gives the carbonate back. On the other hand, a gas G that is
obtained at anode during electrolysis of brine is passed on dry Y, it gives a compound Z,
used for disinfecting drinking water. Identify XXG and Z.
(b) What happens when nitric acid is added to egg shell?
[4]
Solution:
(a) X is calcium carbonate.
CaCO3 + dil. 2HCl 
 CaCl2 + H2O + CO2 
X
Solution Y is lime water Ca(OH)2.
Ca(OH)2 + CO2 
 CaCO3  +H2O
Y
During electrolysis of brine, Cl2 gas is released at anode. So, gas G is Cl2 gas (chlorine
gas).
Ca(OH)2 +Cl2 
 CaOCl2  + H2O
Y
G
Z
Hence, Z is calcium oxychloride (bleaching powder).
(ii) Egg shells are made up of calcium carbonate, CaCO3.
So, brisk effervescence due to the liberation of CO2 gas is observed. The reaction is
CaCO3 +2HNO3 
 Ca(NO3)2 + H2O + CO2 
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