CHEMICAL REACTIONS & EQUATIONS
Intext Ex-1 (Page 6)
Q.1 Why should a magnesium ribbon be cleaned before burning in air?
Ans: Magnesium is very reactive. When stored, it reacts with oxygen to form a
protective layer of magnesium oxide over it. This layer of magnesium oxide is
quite stable and prevents further reaction of magnesium with oxygen. Therefore,
magnesium ribbon is cleaned to remove this magnesium oxide layer before it is
burnt in air.
Q.2 Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate +
Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Ans: (i) H2 + Cl2 -> 2HCl(g)
(ii) 3BaCl2 + Al2(SO4)3 -> 3BaSO4 + 2AlCl3
(iii) 2Na + 2H2O -> 2NaOH + H2
Q.3 Write a balanced chemical equation with state symbols for the
following reactions.
(i) Solutions of barium chloride and sodium sulphate in water react
to give insoluble barium sulphate and the solution of sodium
chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric
acid solution (in water) to produce sodium chloride solution and
water.
Ans: (i)BaCl2(aq) + Na2SO4(aq) -> BaSO4(s) + 2NaCl(aq)
(ii)NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l)
Intext Ex-2 (Page 10)
Q.1 A solution of a substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with
water.
Ans: (i)’X’ is Calcium oxide. Formula is CaO. It is also called quick lime.
(ii) CaO(s) + H2O(l) -> Ca(OH)2 (slaked lime)
Q.2 Why is the amount of gas collected in one of the test tubes in Activity
1.7 double of the amount collected in the other? Name this gas.
Ans: Water contains two part of hydrogen and one part of oxygen. During the
electrolysis of water Hydrogen and Oxygen are collected in separate test tubes.
The test tube in which hydrogen is collected; the amount of gas will be double
of the amount of gas collected in the other.
Intext Ex-3 (Page 13)
Q.1 Why does the colour of copper sulphate solution change when
an iron nail is dipped in it?
Ans: Copper sulphate solution is blue in colour. When iron is dipped in it; iron
being more reactive than copper, displaces copper from the solution to form
Iron sulphate which is green in colour.
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
Q.2 Give an example of a double displacement reaction other than
the one given in Activity 1.10.
Ans: Pb(NO3)2 + 2KI → PbI2 + 2KNO3
Q.3 Identify the substances that are oxidised and the substances
that are reduced in the following reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)
Ans (i) Sodium is oxidised as it gains oxygen and oxygen gets reduced.
(ii) Copper oxide(CuO) is reduced to copper, while hydrogen is oxidised to
water
EXERCISES (Page 14)
Q.1 Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO2(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all
Ans: (i) a and b
Q.2 Fe2O3 + 2Al → Al2O3 + 2Fe
The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.
Ans: (d) displacement reaction
Q.3 What happens when dilute hydrochloric acid is added to iron fillings? Tick
the correct answer.
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced
Ans: (a) Hydrogen gas and Iron chloride are produced
Q.4 What is a balanced chemical equation? Why should chemical equations be
balanced?
Ans: A chemical equation which has equal number of atoms of all the elements
on both sides of the chemical equation is called balanced equation.
A chemical equation should be balanced to follow the law of conservation of
mass which says that mass is neither created nor destroyed in a chemical
reaction. Hence in a chemical reaction the total mass of the reactants should be
equal to the total mass of the products.
Q.5 Translate the following statements into chemical equation and balance
them:
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride
and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and
hydrogen gas.
Ans: (a)3H2(g) + N2(g) → 2NH3(g)
(b) 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)
(c) 3BaCl2(aq) + Al2(SO4)3(aq) → 2AlCl3(aq) + 3BaSO4(s)
(d) 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
Q. 6 Balance the following chemical equations:
Ans: (a)2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b)2NaOH + H2SO4 → Na2SO4 + 2H2O
(c)NaCl + AgNO3 → AgCl + NaNO3
(d)BaCl2 + H2SO4 → BaSO4 + 2HCl
Q.7 Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium
chloride
Ans: (a)Ca(OH)2 + CO2 → CaCO3+ H2O
(b) Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
(c) 2Al + 3CuCl2 → 2AlCl3 + 3Cu
(d) BaCl2 + K2SO4 → BaSO4 + 2KCl
Q.8 Write the balanced chemical equation for the following and identify the
type of reaction in each case.
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) +
Barium bromide(s)
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) +
Hydrogen(g)
Ans: (a)2KBr(aq) + BaI2(aq) → 2KI(aq) + BaBr2(s) (Double displacement)
𝐻𝑒𝑎𝑡
(b)ZnCO3(s) → ZnO(s) + CO2(g) (Thermal Decomposition)
(c) H2(g) + Cl2(g) →2HCl(g) (Combination)
(d)Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) (Displacement)
Q.9 What does one mean by exothermic and endothermic reactions? Give
examples.
Ans: Exothermic Reaction: The chemical reaction during which heat is
evolved.
Examples:
All combustion reactions are exothermic
C + O2 -> CO2 + Heat
CH4 +2O2 -> CO2 + 2H2O+ heat
Respiration is also exothermic
C6H12O6 + 6O2 -> 6CO2 + 6H2O + Energy
Endothermic Reaction: The chemical reaction in which energy is absorbed.
Examples:
Photosynthesis is endothermic
𝑆𝑢𝑛𝑙𝑖𝑔ℎ𝑡
6CO2 + 6H2O → C6H12O6 + 6O2
Q.10 Why is respiration considered an exothermic reaction? Explain.
Ans: Energy is required to support life. Energy for our body is obtained from
food. During respiration, glucose present in our body combines with oxygen in
the cells and provides energy to our body. Since energy is released in the whole
process, hence it is an exothermic reaction.
C6H12O6 + 6O2 -> 6CO2 + 6H2O + Energy
Q.11 Why are decomposition reactions called the opposite of combination
reactions? Write equations for these reactions.
Ans: Decomposition reactions are those in which a compound breaks down to
form two or more simple substances. This reaction requires a source of energy
to take place.
CaCO3 → CaO + CO2
While in combination two or more substances combine to give a single new
substance.
CaO + CO2 → CaCO3
Q.12 Write one equation each for decomposition reactions where energy is
supplied in the form of heat, light or electricity.
Ans: Thermal decomposition reaction: Heat energy is required
𝐻𝑒𝑎𝑡
2FeSO4 → Fe2O3 + SO2 + SO3
Electrolytic decomposition reaction: Electric energy is used
𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦
2H2O → 2H2 + O2
Photolytic decomposition reaction: Light energy is used
𝑆𝑢𝑛𝑙𝑖𝑔ℎ𝑡
2AgCl → 2Ag + Cl2
Q.13 What is the difference between displacement and double displacement
reactions? Write equations for these reactions.
Ans:
Displacement Reaction Double displacement Reaction
1. Reaction in which less 1. Reaction in which ions are
reactive element get exchanged between two
displaced from its compound ionic compounds in their solution.
by more reactive element. BaCl2 + Na2SO4 → BaSO4 + 2NaCl
Zn + CuSO4 → ZnSO4 + Cu
2. Displacement reaction involve 2. This reaction involve reaction
reaction of an element with a between two compounds.
compound.
3. Displacement reaction is 3. May be exothermic or endothermic
exothermic.
4. Displacement reactions are redox 4. Double displacement reactions are
reactions. not redox reactions.
Q.14 In the refining of silver, the recovery of silver from silver nitrate solution
involved displacement by copper metal. Write down the reaction involved.
Ans: Refining of Silver from Silver nitrate:
2AgNO3(aq) + Cu(s) → Cu(NO3)2 + 2Ag(s)
Q.15 What do you mean by a precipitation reaction? Explain by giving
examples.
Ans: Precipitation Reaction: Reaction in which an insoluble product called
precipitate is formed is called precipitation reaction.
Example: When the solution of Barium chloride reacts with the solution of
Sodium sulphate, white precipitate of Barium sulphate is formed along with
Sodium chloride.
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
(white ppt.)
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
(yellow ppt.)
Q.16 Explain the following in terms of gain or loss of oxygen with two examples
each.
(a) Oxidation
(b) Reduction
Ans: Oxidation Reaction: Addition of Oxygen
C + O2 -> CO2
S + O2 ->SO2
Reduction Reaction: Removal of oxygen
ZnO +C -> Zn + CO (ZnO has reduced as it lost oxygen)
CuO + H2 -> Cu + H2O (CuO has reduced as it lost oxygen)
Q.17 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:
Q.18 Why do we apply paint on iron articles?
Ans: When painted, the contact of iron articles with moisture and air is cut off.
Hence rusting is prevented.
Q.19 Oil and fat containing food items are flushed with nitrogen. Why?
Ans: Oxygen reacts with oil and fats present in food, which spoil the taste and
smell of food. This is called Rancidity.
Nitrogen is non-reactive and non-toxic gas and does not react easily with other
substances. This prevents the rancidity of food items. Thus bags used in packing
food items are flushed with nitrogen gas to remove oxygen inside the packets.
Q.20 Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Ans: Corrosion is a chemical process in which the upper layer of metals get
destroyed as a result of chemical reaction with atmospheric gases like oxygen,
water vapour, carbon dioxide and hydrogen sulphide. Due to corrosion metals
lose their lustre and appear dull.
Examples: Rusting of iron
Iron in the presence of moisture reacts with air to form red brown substance
(hydrated iron oxide) commonly called rust.
4Fe + 3O2 + nH2O → 2Fe2O3.nH2O
Copper also get corroded reacting with CO2 present in air and moisture to form
a green coating of basic copper carbonate.
2Cu + O2 + H2O + CO2 → CuCO3.Cu(OH)2
Silver also get corroded reacting with Hydrogen sulphide present in air to Silver
sulphide which is black in colour.
2Ag + H2S → Ag2S + H2
Prevention of corrosion:
1. By coating metal surface with paint
2. By coating with oil or grease
3. By alloying: Eg. Stainless steel is corrosion resistant. It is alloy of iron with
nickel and chromium.
4. By galvanization: Coating a layer of zinc on iron to prevent rusting
5. By electroplating: Coating a layer of corrosion resistant metals like nickel,
chromium and tin on iron by passing electric current.
Rancidity: The process of oxidation of fats and oil present in food that can be
easily noticed by the change in taste and smell is known as rancidity.
Prevention of Rancidity:
1. Storing food in air tight container.
2. Storing food in refrigerators.
3. Adding antioxidants.
4. Storing food in an environment of nitrogen.
