NCERT Solutions for Class 10th Science
Chapter 1 – CHEMICAL REACTIONS AND EQUATIONS
(Complete Downloadable Chapter Solution PDF file is at the bottom of the page)
Q.1 Why should a magnesium ribbon be cleaned before burning in air?
Ans Magnesium being very reactive combines with atmospheric oxygen to form a layer of magnesium oxide which prevents further reaction with oxygen. Hence to remove this oxide layer magnesium ribbon is cleaned before burning 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
(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)
Q.4 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) Substance ‘X’ - Calcium Oxide, also called quick lime. Chemical formula – CaO.
(ii) When quick lime combines with water it forms slaked lime (Calcium hydroxide) with the release of a lot of energy.
CaO(s) + H2O(l) → Ca(OH)2(aq)
(Quick lime) (Slaked lime)
Q.5 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.
Q.6 Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Q.7 Give an example of a double displacement reaction other than the one given in Activity 1.10.
Q.8 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)
EXERCISES
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.
Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)
Q.4 What is a balanced chemical equation? Why should chemical equations be balanced?
Q.5 Translate the following statements into chemical equations and then 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.
Q.6 Balance the following chemical equations.
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
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
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)
Q.9 What does one mean by exothermic and endothermic reactions? Give examples.
Ans Exothermic Reactions: Reactions in which heat is given out along with the products are called exothermic reactions. Eg.
1) CaO(s) + H2O(l) → Ca(OH)2(aq)
(Quick lime) (Slaked lime)
2) Burning of natural gas
CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
Endothermic Reactions: Reactions in which energy is absorbed are known as endothermic reactions.
1) 2AgCl(s) (in sunlight) → 2Ag(s) + Cl2(g)
2) 2Pb(NO3)2(s) (in heat) → 2PbO(s) + 4NO2(g) + O2(g)
Q.10 Why is respiration considered an exothermic reaction? Explain.
Ans During digestion, food is broken down into simpler substances like carbohydrates etc. which further break down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. The process is called respiration. Since energy is released during the process, respiration is an exothermic reaction. The reaction for the same is:
C6H12O6(aq) + 6O2(aq) + 6H2O(l) → 6CO2(aq) + 12H2O(l) + Energy
(Glucose)
Q.11 Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Q.12 Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Q.13 What is the difference between displacement and double displacement reactions? Write equations for these 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.
Q.15 What do you mean by a precipitation reaction? Explain by giving examples.
Q.16 Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction
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.
Q.18 Why do we apply paint on iron articles?
Ans Paint is applied on iron articles to prevent them from rusting. Iron reacts with oxygen and moisture present in the air to form rust i.e. reddish brown coating of ferric oxide (Fe2O3.nH2O) which corrodes the surface. Thus paint isolates iron from moisture and oxygen and prevent rusting.
Q.19 Oil and fat containing food items are flushed with nitrogen. Why?
Ans When fats and oils are oxidised, they become rancid and their smell and taste change. Food items are flushed with nitrogen because it provides an inert environment and thus prevents fats and oils from getting oxidised and become rancid.
Q.20 Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity