# NCERT Solutions

## Class 10 Science

###### chemical reactions and equations

Q1) Which of the statements about the reaction below are incorrect?

$$2PbO(s) + C(s)$$

$$\downarrow$$

$$2Pb(s) + CO_2$$

• (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
###### Solution

a and b
Reason -because oxygen is being removed and removed oxygen is being added to carbon

Q2)The below reaction is an example of a

$$Fe_{2}O_{3} + 2Al \rightarrow Al_{2}O_{3} + 2Fe$$

• (a) combination reaction
• (b) double displacement reaction.
• (c) decomposition reaction
• (d) displacement reaction
###### Solution

d
Displacement reaction

The oxygen from ferrous oxide is getting displaced to aluminum to form aluminum oxide

Q3). 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.
###### Solution

Option - a
The chlorine from the hydrogen chloride is displaced to the iron fillings undergoing following reactions.

$$2HCl + Fe \rightarrow FeCl_{2} + H_{2}$$

Q4). What is a balanced chemical equation? Why should chemical equations be balanced?

###### Solution :

A
balanced chemical equation has an equal number of atoms of different elements in the reactants and products.
Example :

$$Zn +H_{2}SO4 \rightarrow ZnSO_{4} + H_{2}$$

Element Name In Reactants In Products
No. of Zn atoms 1 1
No.of H atoms 2 2
No.of S atoms 1 1
No.of O atoms 4 4

Thus above reaction between zinc and dilute sulphuric acid is a balanced 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 equations should be balanced always.

Q5).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.
###### Solution

$$(a) 3H_{2} + N_{2} \rightarrow 2NH_{3}$$

$$(b) 2H_{2}S + 3O_{2} \rightarrow 2H_{2}O + 2SO_{2}$$

$$(c) 3BaCl_{2} + Al_{2}(SO_{4})_{3}$$

$$\downarrow$$

$$2AlCl_{3} + 3BaSO_{4}$$

$$(d) 2K + 2H_{2}O \rightarrow 2KOH + H_{2}$$

Q6). Balance the following chemical equations

$$(a) \;\;\; HNO_{3} + Ca(OH)_{2}$$

$$\downarrow$$

$$\;\;\; Ca(NO_{3})_{2} + H_{2}O$$

$$(b) \;\;\; 2NaOH + H_{2}SO_{4}$$

$$\downarrow$$

$$\;\;\; Na_{2}SO_{4} + 2H_{2}O$$

$$(c) \;\;\; NaCl + AgNO_{3}$$

$$\downarrow$$

$$\;\;\; AgCl + NaNO_{3}$$

$$(d) \;\;\; BaCl_{2} + H_{2}SO_{4}$$

$$\downarrow$$

$$\;\;\; BaSO_{4} + 2HCl$$

###### Solution :

a) Let's count the number of various atoms in reactants and products.

Element Name In Reactants In Products
No. of H atoms 3 2
No.of N atoms 1 2
No.of C atoms 1 1
No.of O atoms 5 7

In this equation, you have to balance the atoms of both sides, so you start with one type of atom and go from there. Since Oxygen has more atoms in the equation, let's try to balance them first and see if that works. Currently, I have 5 total O atoms (3 + 2) on the left side and 7 on the right (6 + 1) Since they are not balanced, I need to put something on the left to give more on that side. Try 2 HNO3, which gives 6 O atoms, and I had 2 from the Ca(OH)2 already, so I have a total of 8 O atoms now. Right side, you need 8 total, but only have 7. Since H20 has only one O atom, if you place a 2 in front of the H20, you have 2 O atoms now.
Equation so far :

$$\;\;\; 2HNO_{3} + Ca(OH)_{2}$$

$$\downarrow$$

$$\;\;\; Ca(NO_{3})_{2} + 2H_{2}O$$

Again, balance all the atoms for reactants and products. Reactants:
• 2HNO3 has 2 H atoms, 2 N atom and 6 O atoms
• Ca(OH)2 has 1 Ca atom, 2 O atoms and 2 H atoms
• Left side: 4 H atoms, 2 N atoms, 8 O atoms, 1 Ca atom

Products:
Ca(NO3)2 has 1 Ca atom, 2 N atoms, 6 O atoms
2H2O has 4 H atoms (2 x 2), and 2 O atoms
Right side: 1 Ca atom 2 N atoms, 8 O atoms, 4 H atoms
Since the numbers on the left equal the right, you now have a balanced equation, so this must be the answer:

$$\;\;\; HNO_{3} + Ca(OH)_{2}$$

$$\downarrow$$

$$\;\;\; Ca(NO_{3})_{2} + H_{2}O$$

$$(b) \;\;\; NaOH + H_{2}SO_{4}$$

$$\downarrow$$

$$\;\;\; Na_{2}SO_{4} + H_{2}O$$

$$(c) \;\;\; NaCl + AgNO_{3}$$

$$\downarrow$$

$$\;\;\; AgCl + NaNO_{3}$$

$$(d) \;\;\; BaCl_{2} + H_{2}SO_{4}$$

$$\downarrow$$

$$\;\;\; BaSo_{4} + HCl$$

Q7). 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

$$(a) Ca(OH)_2 + CO_{3} \rightarrow CaCO_{3} + H_{2}O$$

$$(b) 2AL + 3Cu(Cl)_2 \rightarrow AlCl_{3} + 3Cu$$

$$(c) \;\;\; Zn(s) + 2AgNO_{3}(s)$$

$$\downarrow$$

$$\;\;\; Zn(NO_{3})_{2} + 2Ag$$

$$(d) BaCl_{2} + K_{2}SO_{4} \rightarrow BaSO_{4} + 2KCl$$

Q8). 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) gives Hydrogen chloride(g)
• (d) Magnesium(s) + Hydrochloric acid(aq)givesMagnesium chloride(aq) + Hydrogen(g)
###### Solution

(a)

$$(a) \;\;\; 2KBr(aq) + BaI_{2}(aq)$$

$$\downarrow$$

$$\;\;\; 2KI(aq) + BaBr_{2}$$

It is a double displacement reaction
$$(b) ZnCO_{3}(s) \rightarrow ZnO(s) + CO_{2}(g)$$
It is a decomposition reaction.
$$(c) H_{2}(g) + Cl_{2}(g) \rightarrow 2HCl$$
It is a combination reaction

$$(d) \;\;\; Mg(s) + 2HCl(aq)$$

$$\downarrow$$

$$\;\;\; MgCl_{2} (aq) + H_{2}(g)$$

It is a displacement reaction.

Q10). Why is respiration considered an exothermic reaction ? Explain.

###### Solution

In the course of respiration glucose is broken into carbon dioxide and energy is released. That’s why it is considered as an exothermic reaction.

$$\;\;\; C_{6}H_{12}O_{6}(aq) + 6O_{2}(g)$$

$$\downarrow$$

$$\;\;\; 6CO_{2}(aq) + 6H_{2}O (L) + Energy$$

Q11) Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

###### Solution

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
$$CaCO_{3} (S) \rightarrow CaO (s) + CO_{2}(g)$$
This is a Decomposition reaction $$H_{2}(g) + Cl_{2} \rightarrow 2HCl (g)$$
This is a Combination reaction

Q12) Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

###### Solution

$$CaCO_{3} (s) \rightarrow CaO (s) + CO_{2} (g)$$
In this reaction energy is supplied in the form of heat.
$$2AgCl (s) + sunlight \rightarrow 2Ag(s) + Cl_{2}(g)$$
In this reaction energy is supplied in the form of light,i.e.sunlight $$2H_{2}O + electricity \rightarrow 2H_{2} + O_{2}$$
In this reaction energy is supplied in the form of electricity.

Q13) What is the difference between displacement and double displacement reactions? Write equations for these reactions.

###### Solution

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 takes place. Following are examples of displacement reaction and double displacement reaction.

$$Mg(s) + 2HCl(aq) \rightarrow MgCl_{2} + H_{2}(g)$$

$$\;\;\; 2KBr(aq) + BaCl_{2}$$

$$\downarrow$$

$$\;\;\; 2KCl(aq) + BaBr_{2}$$

Q14) In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

###### Solution

$$\;\;\; 2AgNO_{3}(aq) + Cu(s)$$

$$\downarrow$$

$$\;\;\; Cu(NO_{3})_{2} + 2Ag(s)$$

The silver nitrate is in solution and the metallic copper will dissolve to form copper nitrate; as it does, it will precipitate the silver in solution as metallic silver. That is, copper is exchanged for the silver in solution and silver is replaced by the copper that is not in solution.

Q15) What do you mean by a precipitation reaction? Explain by giving examples.

###### Solution

Reactions in which precipitate is formed are called precipitation reactions. Generally in a precipitation reaction insoluble salts are settled down as precipitate.

$$Na_{2}SO_{4} (aq) + BaCl_{2} (aq)$$

$$\downarrow$$

$$2Nacl (aq) + BaSO_{4}(s)$$

In the above reaction, when sodium sulphate reacts with barium chloride, the white precipitate of barium sulphate is formed along with the sodium chloride.

Q16) Explain the following: (a) Oxidation (b) Reduction

###### Solution

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. In the following reaction, copper gets oxygen and hence is oxidised.
$$2Cu + O_{2} +Heat \rightarrow 2CuO$$
$$CuO + H_{2} + Heat \rightarrow Cu + H_{2}O$$
In the above reaction hydrogen gains oxygen, and hence oxidized. Reduction: If a substance loses oxygen or gains hydrogen during a reaction, it is that the substance is reduced. This phenomenon is called reduction. In the following reaction zinc loses oxygen, and hence called is reduced.
$$ZnO + C \rightarrow ZnO$$ $$MnO_{2} + 4HCl \rightarrow MnCl_{2} + 2H_{2}O + Cl_{2}$$ In this reaction Mangnese dioxide loses oxygen and hence is called reduced.

Q17) shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

###### Solution

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

Q18) Why do we apply paint on iron articles?

###### Solution

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 to come in contact with moisture present in air paint is applied. Paint prevents the iron to get rusted.

Q19) Oil and fat containing food items are flushed with nitrogen. Why?

###### Solution

When fats and oils are oxidised, they become rancid and their smell and taste change. Hence, food items containing oil and fat are flushed with nitrogen which prevents them to get oxidized and becoming rancid.