Class 10 Science Chapter 1 Chemical Reactions and Equations

Chapter 1. Chemical Reactions and Equations

NCERT Solution for Class 10 Science Chapter 1 Exercise - 1

Question (1)

Why should a magnesium ribbon be cleaned before burning in air?

Answer :

Magnesium is very reactive metal. When stored it reacts with oxygen to form a layermagnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper to remove this layer so that the underlying metal can be exposed into air.

Question (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

Answer :

(i) H_2(g)+ Cl_2(g) →→ 2HCL_(g)

(ii) 3BaCl_2(s) + Al₂(SO₄)_3(s) →→ 3BaSO_4(s) + 2AlCl_3(s)

(iii) 2Na_(s) + 2H₂O_(l) →→ 2NaOH_(aq) + H_2(g)

Question (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.

Answer :

(i) BaCl_2(aq) + Na₂SO_4(aq) →→ BaSO_4(s) + 2NaCl_(aq)

(ii) NaOH_(aq) + HCl_(aq) →→ NaCl_(aq) + H₂O_(l)

NCERT Solution for Class 10 Science Chapter 1 Exercise - 2

Question (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.

Answer :

The substance ‘X’ is calcium oxide. Its chemical formula is CaO.Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).

CaO_(s) + H₂O_(l) →→ Ca(OH)_2(aq)

Calcium Oxide Water Calcium Hydroxide

(Quick Lime) (Slaked lime)

Question (2)

Why is the amount of gas collected in one of the test tubes in Activity 1.7 double ofthe amount collected in the other? Name this gas.

Answer :

Water (H₂O) contains two parts hydrogen and one part oxygen. Therefore, the amountof hydrogen and oxygen produced during electrolysis of water is in a 2:1 ratio. During electrolysis, since hydrogen goes to one test tube and oxygen goes to another, the amount of gas collected in one of the test tubes is double of the amount collected in the other.

NCERT Solution for Class 10 Science Chapter 1 Exercise - 3

Question (1)

Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer :

When an iron nail is placed in a copper sulphate solution, iron displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.

Fe_(s) + CuSO_4(aq) →→ FeSO_4(aq) + Cu_(s)

Iron Copper Iron Copper

Sulphate(Blue colour) Sulphide

Therefore, the blue colour of copper sulphate solution fades and green appears.

Question (2)

Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer :

Sodium Carbonate reacts with calcium chloride to form calcium carbonate and sodiumchloride.

Na2CO_3(aq) + CaCl_2(aq) →→ CaCO_3(s) + 2NaCl_(aq)

Sodium carbonate Calcium chloride Calcium Carbonate Sodium

Chloride

In this reaction, sodium carbonate and calcium chloride exchange ions to form two new compounds. Hence, it is a double displacement reaction.

Question (3)

Identify the substances that are oxidised and the subsatnces that are reduced in the following reactions.

(i) 4Na_(s) + O_2(g) →→ 2Na₂O_(s)

(ii) CuO_(s) + H_2(g) →→ Cu_(s) + H₂O_(l)

Answer :

(i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.

(ii) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H₂) gets oxidised to water (H₂O).

NCERT Solution for Class 10 Science Chapter 1 Exercise - 4

Question (1)

Which of the statements about the reaction below are incorrect?

2PbO_(s) + C_(s) →→ 2Pb_(s) + CO_2(g)

(a) Lead is getting reduced.

(b) Carbon dioxide is getting oxidised.

(c) Carbon is getting oxidised.

(d) Lead oxide is getting reduced.

OPTIONS:

(i) (a) and (b)

(ii) (a) and (c)

(iii) (a), (b) and (c)

(iv) all

Answer :

(i) (a) and (b)

Question (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.

Answer :

(d) The given reaction is an example of a displacement reaction.

Question (3)

What happens when dilute hydrochloric acid is added to iron filings? 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.

Answer :

Hydrogen gas and iron chloride are produced. The reaction is as follows:

Fe_(s) + 2HCl_(aq) FeCl_2(aq) + H2 ↑

Question (4)

What is a balanced chemical equation? Why should chemical equations bebalanced?

Answer :

A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation. The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in a chemical reaction, the total mass of reactants should be equal to the total mass of the products. It means that the total number of atoms of each element should be equal on both sides of a chemical equation. Hence, it is for this reason that chemical equations should be balanced.

Question (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 precipitate of barium sulphate.

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas

Answer :

(a) 3H_2(g) + N_2(g) →→ 2NH_3(g)

(b) 2H₂S_(g) + 3O_2(g) →→ 2H₂O_(l) + 2SO_2(g)

(c) 3BaCl_2(aq)+ Al₂(SO₄)_2(aq)→→ 2AlCl_3(aq)+ 3BaSO_4(s)

(d) 2K_(s) + 2H₂O_(l) →→ 2KOH_(aq) + H_2(g)

Question (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

Answer :

(a) 2HNO₃ + Ca(OH)₂ →→ Ca(NO₃)₂ + 2H₂O

(b) 2NaOH + H₂SO₄ →→ Na₂SO₄ + 2H₂O

(c) NaCl + AgNO₃ →→ AgCl + NaNO₃

(d) BaCl₂+ H₂SO₄→→ BaSO₄ + 2HCl

Question (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

Answer :

(a) Ca(OH)₂ + CO₂ →→ CaCO₃ + H₂O

(b) Zn + 2AgNO3 →→ Zn(NO3)2 + 2Ag

(c) 2Al + 3CuCl₂ →→ AlCl₃ + 3Cu

(d) BaCl₂ + K₂SO₄ →→ BaSO₄ + 2KCl

Question (8)

Write the balanced chemical equation for the following and identify the type ofreaction 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)

Answer :

(a) 2KBr_(aq) + Bal_2(aq) →→ 2Kl_(aq) + BaBr_2(s) ; Double displacement reaction

(b) ZnCO_3(s) →→ ZnO_(s) + CO_2(g) ; Decomposition reaction

(c) H2(g) + Cl2(g) →→ 2HCl(g) ; Combination reaction

(d) Mg_(s) + 2HCl_(aq) →→ MgCl_2(aq) + H_2(g) ; Displacement reaction

Question (9)

What does one mean by exothermic and endothermic reactions? Give examples.

Answer :

Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions.

Example: Mixture of sodium and chlorine to yield table salt

Na_(s) + 1212 Cl_2(s) →→ NaCl_(s) + 411kJ of energy

In other words, combination reactions are exothermic.

Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.

For example: In the process of photosynthesis, plants use the energy from the sun to convert carbon dioxide and water to glucose and oxygen.

Sunlight

6CO_2(g) + 6H₂O_(l) →→ C₆H₁₂O_6(aq) + 6O_2(g)

Question (10)

Why is respiration considered an exothermic reaction? Explain.

Answer :

Energy is required to support life. Energy in our body is obtained from the food we eat. During digestion, large molecules of food are broken down into simpler substances such as glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this combustion reaction is respiration. Since energy is released in the whole process, it is an exothermic process.

C₆H₁₂O_6(aq) + 6O_2(g) →→ 6CO_2(g) + 6H₂O_(l) + Energy

Glucose Oxygen Carbon-dioxide Water

Question (11)

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

Answer :

Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy.

Decomposition reaction: AB + Energy →→ A + B

2H₂O_(l) →→ 2H_2(g) + O_2(g)

Combination reaction: A + B →→ AB + Energy

2H_2(g) + O_2(g) →→ 2H₂O_(l) + Energy

Question (12)

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

Answer :

(a) Thermal decomposition:

2FeSO_4(s) →→ Fe₂O_3(s) + SO_2(g) + SO_3(g)

Ferrous Sulphide Ferric oxide Suphur dioxide Sulphur trioxide

(b) Decomposition by light:

2AgCl_(s) →→ 2Ag_(s) + Cl_2(g)

Silver Chloride Silver Chlorine

2Al₂O_3(aq) →→ 4Al_(s)+ 3O_2(g)

Aluminium oxide Aluminium Oxygen

Question (13)

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

Answer :

In a displacement reaction, a more reactive element replaces a less reactive element from a compound.

A + BX AX + B; where A is more reactive than B

In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds.

AB + CD →→ AD + CB

For example:

Displacement reaction:

CuSO_4(aq) + Zn_(s) →→ ZnSO_4(aq) + Cu_(s)

Double displacement reaction:

Na₂SO_4(aq) + BaCl_2(aq) →→ BaSO_4(s) + 2NaCl_(aq)

Question (14)

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

Answer :

2AgNO_3(aq) + Cu_(s)→→ Cu(NO₃)_2(aq) + 2Ag_(s)

Silver Nitrate Copper Copper nitrate Silver

Question (15)

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

Answer :

A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.

For example:

Na₂CO_3(aq) + CaCl_2(aq) →→ CaCO_3(s) + 2NaCl_(aq)

Sodium Carbonate Calcium Chloride Calcium Carbonate Sodium

Chloride

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.

Another example of precipitation reaction is:

Na₂SO₄ + BaCl_2(aq) →→ BaSO_4(s) + 2NaCl_(aq)

Sodium Sulphate Barium Chloride Barium Sulphate Sodium Chloride

In this reaction, barium sulphate is obtained as a precipitate.

Question (16)

Explain the following in terms of gain or loss of oxygen with two examples each.

(a) Oxidation

(b) Reduction

Answer :

(a) Oxidation is the gain of oxygen.

For example:

(i) CO₂ + H₂ →→ CO + H₂O

Addition of oygen-oxidation

(ii) 2Cu + O₂→→ 2CuO

Gain of oxygen - oxidation

In equation (i), H₂ is oxidized to H₂O and in equation (ii), Cu is oxidised to CuO.

(b) Reduction is the loss of oxygen.

For example:

(i) CO₂ + H₂→→ CO + H₂O

Reduction of oxygen - reduction

(ii) CuO + H₂ →→ Cu + H₂O

Loss of oxygen - reduction

In equation (i), CO₂is reduced to CO and in equation (ii), CuO is reduced to Cu.

Question (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.

Answer :

'X' is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below.

2Cu + O2 →→ 2CuO

(Shiny brown in colour) (Black in colour)

Question (18)

Why do we apply paint on iron articles?

Answer :

Iron particles are painted because it prevents them from rusting. When painted, the contact of iron particles from moisture and air is cut off. Hence, rusting is prevented their presence is essential for rusting to take place.

Question (19)

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

Answer :

Nitrogen is an inert gas and does not easily react with these substances. On the otherhand, oxygen reacts withfood substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat containing food items is avoided.

Question (20)

Explain the following terms with one example each

(a) Corrosion

(b) Rancidity

Answer :

(a) Corrosion:

Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.

For example, iron in the presence of moisture, reacts with oxygen to form hydrated iron oxide.

This hydrated iron oxide is rust.

(b) Rancidity:

The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.

For example, the taste and smell of butter changes when kept for long.