## NCERT solutions for class 10 science-Exercise Solutions

Reviewed By:
Krishna Kant Majee
M.Sc., B.Ed.

### Chapter 01-Chemical Reactions and Equations-Exercise Solutions

#### NCERT Book Page Number-14

Q.1. Which of the statements about the reaction below are incorrect?
$2PbO(s) + C(s)\rightarrow 2Pb(s) + CO_{2}(g)$
(b) Carbon dioxide is getting oxidized.
(c) Carbon is getting oxidized.
(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.$Fe_{2}O_{3}(s) + 2Al(s)\rightarrow Al_{2}O_{3}(s) + 2Fe(l)$
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. When the number of atoms of reactants is equal to the number of atoms of the product, the reaction is called a balanced chemical equation. According to the law of conservation of mass, total mass of the elements present in the reactants must be equal to the total mass of the elements present in products. That’s why a chemical equation should be balanced always.

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 sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a pre¬cipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Ans. (a)$3H_{2}(g)+ N_{2}(g)\rightarrow 2NH_{3}(g)$
(b) $2H_{2}S + 3O_{2}\rightarrow 2H_{2}O + 2SO_{2}$
(c) $3BaCl_{2}(aq) + Al_{2}(SO_{4})_{3}(aq)\rightarrow 2AlCl_{3}(aq) + 3BaSO_{4}(s)$
(d) $2K(s) + 2H_{2}O(l) → 2KOH (aq)+ H_{2}$

Q.6. Balance the following chemical equations :
(a) $HNO_{3} + Ca(OH)_{2} → Ca(NO_{3})_{2} + H_{2}O$
(b) $NaOH + H_{2}SO_{4} → Na_{2}SO_{4} + H_{2}O$
(c) $NaCl + AgNO_{3}\rightarrow AgCl + NaNO_{3}$
(d) $BaCl_{2} + H_{2}SO_{4}\rightarrow BaSO_{4} + HCl$
Ans. (a) $2HNO_{3} + Ca(OH)_{2}\rightarrow Ca(NO_{3})_{2} + 2H_{2}O$
(b) $2NaOH + H_{2}SO_{4}\rightarrow Na_{2}SO_{4} + 2H_{2}O$
(c) $NaCl + AgNO_{3}\rightarrow AgCl + NaNO_{3}$
(d) $BaCl_{2} + H_{2}SO_{4}\rightarrow BaSO_{4} + 2HCl$

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) $Ca(OH)_{2} + CO_{2}\rightarrow CaCO_{3} + H_{2}O$
(b) $Zn + 2AgNO_{3}\rightarrow Zn(NO_{3})_{2} + 2Ag$
(c) $2Al + 3CuCl_{2}\rightarrow 2AlCl_{3} + 3Cu$
(d) $BaCl_{2} + K_{2}SO_{4}\rightarrow BaSO_{4} + 2KCl$

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. $(a) 2KBr(aq) + BaI_{2}\rightarrow 2KI(aq) + BaBr_{2}(s)$
It is a double displacement reaction.
(b) $ZnCO_{3}(s)\rightarrow ZnO(s) + CO_{2}(g)$
It is a decomposition reaction.
(c) $H_{2}(g) + Cl_{2}(g)\rightarrow 2HCl(g)$
It is a combination reaction.
(d) $Mg(s) + 2HCl(aq)\rightarrow MgCl_{2}(s) + H_{2}(g)$
It is a displacement reaction.

Q.9. What does one mean by exothermic and endothermic reactions? Give examples.
Ans. Exothermic reaction : A reaction in which heat is released is called the exothermic reaction. Burning of fuel is an example of exothermic reaction. When methane is burnt it gives heat along with carbon dioxide and water.
$CH_{4}(g) + 2O_{2}(g)\rightarrow CO_{2}(g) + 2H_{2}O(l) + Heat$
Endothermic reaction : A reaction in which heat is absorbed is called endothermic reaction.
Example : When silver chloride is left in the sunlight, it absorbs heat and turns grey because of formation of silver metal.
$2AgCl(s)\rightarrow2Ag(s) + Cl_{2}(g)$

Q.10. Why is respiration considered an exothermic reaction? Explain.
Ans. In the course of respiration glucose is broken into water and carbon dioxide and energy is released. That is why it is considered as an exothermic reaction.
$C_{6}H_{12}O_{6}(aq) + 6O_{2}(g)\rightarrow 6CO_{2}(aq) + 6H_{2}O(l) + Energy$

Q.11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Ans. In a combination reaction two or more substances are combined to form new substance while in the decomposition reaction one substance decomposes into two or more substances. Hence, the decomposition reactions are called opposite of combination reactions.
$CaCO_{3}(s)\rightarrow CaO(s) + CO_{2}(g) ~~~ Decomposition ~~~reaction$
$H_{2}(g) + Cl_{2}\rightarrow2HCl(g) – Combination reaction$

Q.12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Ans. $CaCO_{3} \rightarrow CaO(s) + CO_{2}(s)$
In this reaction energy is supplied in the form of heat.
$2AgCl(s) \rightarrow 2Ag(s) + Cl_{2}(g)$
In this reaction energy is supplied in the form of light, i.e., sunlight.
$2H_{2}O(l)\rightarrow 2H_{2}(g) + O_{2}(g)$
In this reaction energy is supplied in the form of electricity.

Q.13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Ans. When a more reactive element displaces a less reactive element from its salt solution, then it is called displacement reaction. While there is an exchange of ions between the reactants to produce new substances, it is called double displacement reaction. In displacement reaction one displacement takes place. While in double displacement reaction two displacements take place.
Displacement reaction
$Mg(s) + 2HCl(aq)\rightarrow MgCl_{2} + H_{2}(g)$
Double displacement reaction
$2KBr(aq) + BaI_{2}(s) \rightarrow 2KI(aq) + BaBr_{3}(s)$

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. $2AgNO_{3}(aq) + Cu(s)\rightarrow Cu(NO_{3})_{2}(l) + 2Ag(s)$

Q.15. What do you mean by a precipitation reaction? Explain by giving examples.
Ans. Reactions in which precipitate is formed are called precipitation reactions. In which one of the product is insoluble. Generally in a precipitation reaction insoluble salts are settled down as precipitate.
$Na_{2}SO_{4}(aq) + BaCl_{2}(aq)\rightarrow BaSO_{4}(s) + 2NaCl(aq)$
In the above reaction, when sodium sulphate reacts with barium chloride, the white precipitate of barium sulphate is formed along with the sodium chloride.

Q.16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation (b) Reduction
Ans. (a) Oxidation : When a substance gains oxygen or loses hydrogen during a reaction, it is called that the substance is oxidized. This phenomenon is called oxidation.
$2Cu(s) + O_{2}(g) \rightarrow 2CuO(s)$
In this reaction copper gains oxygen and hence is oxidized.
$CuO + H_{2} \rightarrow Cu + H_{2}O$
In the above reaction hydrogen gains oxygen, and hence oxidized.
(b) Reduction : If a substance loses oxygen or gains hydrogen during a reaction, it is called that the substance is reduced. This phenomenon is called reduction.
$ZnO + C \rightarrow Zn + CO$
In this reaction zinc loses oxygen, and hence is reduced.
$MnO_{2} + 4HCl\rightarrow MnCl_{2} + 2H_{2}O + Cl_{2}$
In this reaction manganese dioxide loses oxygen and hence is called reduced.

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. Copper is a brown coloured shiny element, when heated in air, it becomes black in colour.
$2Cu(s) + O_{2}(g) \rightarrow 2CuO()$
Hence, the element ‘X’ is copper and black coloured compound formed is copper oxide.

Q.18. Why do we apply paint on iron articles?
Ans. When articles made of iron come in contact with the moisture present in air, it forms iron oxide, which is known as rust. Hence, to prevent an iron article coming in contact with moisture present in air, paint is applied. Paint prevents the iron to get rusted.

Q.19. Oil and fat containing food items are flushed with nitrogen. Why?
Ans. When fats and oils are oxidised, they become rancid causing a change in smell and taste. Hence, food items containing oil and fat are flushed with nitrogen which prevents them to get oxidized and becoming rancid.

Q.20. Explain the following terms with one example of each.
(a) Corrosion (b) Rancidity

Ans. (a) Corrosion : Metals react with oxygen which is present in the atmospheric moisture. This leads to the formation of metal oxides. In due course of time, the metal keeps on changing into its oxide and finally the whole metal is lost due to oxidation. This process is called corrosion.
Example : Iron articles ; like iron gates or bridges tend to rust because of oxidation by atmospheric moisture. We know that rust is nothing but iron oxide. Conversion of iron into rust leads to corrosion of the iron articles. Due to this the iron articles weakens and finally wither away.
(b) Rancidity : When fats and oils are oxidised, their smell and taste change. This process is called rancidity. Oily food often become rancid and start giving out obnoxious smell. The taste also becomes bad. Such oily food is not fit for eating.
Example : When packets of potato chips or other oily snacks are kept open for a long time; their taste and smell become bad. The oily food is no longer safe to eat.