SELINA Solutions for Class 9 Chemistry Chapter 2 - Chemical Changes and Reactions

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Chapter 2 - Chemical Changes and Reactions Exercise Ex. 2(A)

Question 1

(a) What is a chemical reaction?

(b) State the conditions necessary for a chemical change or reaction.

Solution 1

(a) A chemical reaction is the process of breaking the chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products).

(b) Conditions necessary for a chemical change or reaction are

     (i) Evolution of gas

    (ii) Change of colour

    (iii) Formation of precipitate

    (iv) Change of state

Question 3

Give an example of a reaction where the following are involved

(a) Heat

(b) Light

(c) Electricity

(d) Close contact

(e) Solution

(f) Pressure

(g) Catalyst

Solution 3

(a)   


(b)   


(c)   


(d)   


(e)   


(f)   


(g)   

Question 4

Define

(a) Photochemical reaction

(b) Electrochemical reaction

Give an example in each case.

Solution 4

(a) It is a reaction which occurs with absorption of light energy.

       

 

(b) It is a reaction which occurs with absorption of electrical energy.

       

 

Question 5

Give an example of each of the following chemical changes: 

  1. A photochemical reaction involving 

(i) silver salt (ii) water 

  1. A reaction involving 

(i) blue solution 

(ii) formation of dirty green precipitate 

  1. Two gases combine to form white solid
  2. Two solids combine to form a liquid
  3. A reaction where colour change is noticed
Solution 5

a.  

 i.   

 ii.   

 

 

b.  

 i. 

 

 

 ii. 

c.   

d.   

 

e. 

Question 6

Write the chemical reaction where the following changes are observed.

(a) Gas is evolved

(b) Colour change is noticed

(c) Precipitate is formed

(d) Physical state is changed

Solution 6

(a)   


  


(c)   


(d)   

Question 7

Give reason for the following:

  1. Silver nitrate solution is kept in coloured bottles.
  2. Molybdenum is used in the manufacture of ammonia.
  3. Blue solution of copper sulphate changes to green when a piece of iron is added to this solution.
  4. Colourless concentrated sulphuric acid in a test tube changes to blue on adding a small piece of copper to it.
Solution 7
  1. Silver nitrate solution is kept in brown bottles in the laboratory because it decomposes in the presence of light.
  2. Molybdenum increases the efficiency of the catalyst iron used in the manufacture of ammonia.
  3. This is because the blue colour of the copper sulphate solution fades and eventually turns into light green due to the formation of ferrous sulphate.
  4. Copper displaces hydrogen from sulphuric acid and forms blue-coloured copper sulphate and hydrogen gas is evolved.
Question 2

Define the following terms 

  1. Chemical change 
  2. Chemical bond 
  3. Effervescence 
  4. Precipitate
Solution 2
  1. A chemical change is a permanent change in which the composition of a substance which results in the formation of substance with different chemical composition and properties. 
  2. A chemical bond is the force which holds the atoms of a molecule together as in a compound. 
  3. Formation of gas bubbles in a liquid during a reaction is called effervescence. 
  4. Chemical reactions which are characterised by the formation of insoluble solid substances are called precipitates. 

Chapter 2 - Chemical Changes and Reactions Exercise Ex. 2(B)

Question 1

Complete the following statements:

  1. The chemical change involving iron and hydrochloric acid illustrates a _________________ reaction.
  2. In the type of reaction called_______________, two compounds exchange their positive and negative radicals.
  3. A catalyst either ______ or _____________ the rate of a chemical change but itself remains ______________ at the end of the reaction.
  4. On heating, hydrated copper sulphate changes its colour from ________ to __________.
Solution 1
  1. Displacement
  2. Double decomposition
  3. Accelerates, decelerates, unaffected
  4. Blue, white
Question 2

When hydrogen burns in oxygen, water is formed; when electricity is passed through water, hydrogen and oxygen are given out. Name the type of chemical change involved in the two cases.

Solution 2

When hydrogen burns in oxygen, water is formed -  Combination Reaction.

When electricity is passed through water, hydrogen and oxygen are given out - Decomposition Reaction.

Question 3

Explain, giving one example for each of the following chemical changes:

(a) Double decomposition

(b) Thermal decomposition

(c) Reversible reaction

(d) Displacement

Solution 3

(a) Double decomposition: This is a type of chemical change in which two compounds in a solution react to form two new compounds by mutual exchange of radicals.

  

 

(b) Thermal decomposition

A decomposition reaction brought about by heat is known as thermal decomposition.

2HgO(s)  2Hg(s) +O2(g) 

 

(c) Reversible reaction

A chemical reaction in which the direction of a chemical change can be reversed by changing the conditions under which the reaction is taking place is called a reversible reaction.

CuSO4.5H2O(s) ⇋ CuSO4(s) + 5H2O(g) 

 

(d) Displacement

It is a chemical change in which a more active element displaces a less active element from its salt solution.

CuSO4 + Zn ZnSO4 + Cu 

Question 4

(a) What is synthesis?

(b) What kind of chemical reaction is synthesis? Support your answer by an example.

Solution 4

A reaction in which two or more substances combine together to form a single substance is called a synthesis or combination reaction.

A + B  AB

In the above reaction, substances A and B combine to give a molecule of a new substance, AB.

Carbon burns in oxygen to form a gaseous compound, carbon dioxide.

C + O2 CO2 

Question 5

Decomposition brought about by heat is known as thermal decomposition. What is the difference between thermal dissociation and thermal decomposition?

Solution 5

A decomposition reaction brought about by heat is known as thermal decomposition.

2HgO(s)  2Hg(s) + O2(g)

A simultaneous reversible decomposition reaction brought about only by heat is thermal dissociation.

NH4Cl  NH3 +HCl

Question 6
  1. Define neutralization reaction with an example.
  2. Give balanced equation for this reaction.
  3. Give three applications of neutralization reactions.
Solution 6
  1. The reaction between an acid and a base to form salt and water only is referred to as a neutralisation reaction.

 

  1. NaOH + HCl NaCl + H2O
  2. Applications of neutralisation reactions:
    1.  When someone is stung by a bee, formic acid enters the skin and causes pain, which can be relieved by rubbing the spot with slaked lime or baking soda, both of which are bases.
    2.  Acid which is accidentally spilled on to our clothes can be neutralised with ammonia solution.
    3.  If soil is somewhat acidic and thus unfavourable for growing of certain crops, slaked lime is added to neutralise the excess acid.
Question 7

What do you understand by precipitation reaction? Explain with an example.

Solution 7

Precipitation reaction 

The insoluble solid formed during the double displacement reactions is called a precipitate. Reactions in which a precipitate is formed as one of the products are also called precipitation reactions.

Sodium sulphate reacts with barium chloride to form barium sulphate and sodium chloride solution.

Na2SO4 (aq) + BaCl2 → BaSO4(s) + 2NaCl (aq) 

Sodium Barium Barium Sodium 

sulphate chloride sulphate chloride 

Question 8
  1. What are double displacement reactions?
  2. Give me example of double displacement reaction, where a gas is evolved.
Solution 8
  1. Reactions in which two compounds react with each other and exchange their ions to form two new compounds, are called double displacement reaction. In these reactions, two compounds swap components, in the format:

AB + CD → AD + CB

  1. The reactants of double displacement reaction are in aqueous form i.e. solution and are often accompanied by precipitation. In other words, a solid product separates or settles at the bottom of the solution.

FeS(s) + H2SO4 (aq) FeSO4(aq) + H2S↑ is a an example of double displacement reaction, where a gas is evolved.

Question 9

(a) What is decomposition?

(b) Support your answer by an example.

Solution 9

Decomposition is the breaking up of a compound either into elements or simpler compounds such that these products do not combine to form the original compound.

Decomposition may occur in the presence of heat or light or by the passage of an electric current.

Example: Mercuric oxide when heated decomposes to form two elements-mercury and oxygen.

2HgO(s)  2Hg(s) + O2(g) 

Question 10

State the type of reactions each of the following represent and balance the ones that are not balanced.

(a) Cl2 + 2KBr 2KCl + Br2 

(b) NaOH + HCl NaCl + H2O 

(c) 2HgO 2Hg + O2 

(d) Fe + CuSO4FeSO4 + Cu 

(e) PbO2 + SO2PbSO4 

(f) 2KClO32KCl + 3O2 

(g) 2H2O22H2O + O2 

(h) KNO3 + H2SO4 HNO3 + KHSO4 

(i) CuO+H2Cu+ H2O 

(j) CaCO3CaO+ CO2  

(k) NH4Cl NH3 + HCl 

(l) PbO + 2HNO3 Pb(NO3) + 2H2O 

(m) AgNO3 + NaCl AgCl + NaNO3 

Solution 10
  1. Cl2 + 2KBr → 2KCl + Br2
    Displacement reaction
  1. NaOH + HClNaCl + H2O
    Neutralisation reaction
  1. 2HgO → 2Hg + O2
    Decomposition reaction
  1. Fe + CuSO4 → FeSO4 + Cu
    Displacement reaction
  1. PbO2 + SO2 → PbSO4
    Combination reaction
  1. 2KClO3 → 2KCl + 3O2
    Decomposition reaction
  1. 2H2O2 → 2H2O + O2
    Decomposition reaction
  1. KNO3 + H2SO4→ HNO3 + KHSO4
    Double decomposition reaction
  1. CuO+H2 → Cu+ H2O
    Displacement reaction
  1. CaCO3CaO+ CO2
    Decomposition reaction
  1. NH4Cl → NH3 + HCl 
    Decomposition reaction 
  1. PbO + 2HNO3Pb(NO3) + 2H2O
    Neutralisation reaction
  1. AgNO3 + NaCl → AgCl + NaNO3
    Double decomposition reaction
Question 11

Match the following:

a. Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)

 i. Photochemical decomposition

b. 

 ii. Thermal decomposition

c. 

iii. Displacement reaction

d.   

iv. Electrolytic decomposition

 

Solution 11

 

a. Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)

 i. Displacement reaction

b. 

 ii. Photochemical decomposition

c. 

iii. Electrolytic decomposition

d.   

iv. Thermal decomposition

 

Question 12

Multiple choice:

a. Which of the following is not a characteristic of a chemical change?

 i. It is irreversible.

 ii. No net energy change is involved.

 iii. New substance is formed.

 iv. Involves absorption or liberation of energy.

 

b. A reaction of a type: AB + CD → AD + CD, involves 

 i. No chemical change

 ii. Decomposition of AB and CD

 iii. Exchange of ions of AB and CD

 iv. Combination of AB and CD

 

c. The reaction BaCl2(aq) + H2SO4(aq) → BaSO4(s) + 2HCl(aq) is

 i. Displacement reaction

 ii. Neutralisation reaction

 iii. Decomposition reaction

 iv. Double displacement reaction

 

d. Thermal decomposition of sodium carbonate will produce

 i. Carbon dioxide

 ii. Oxygen

 iii. Sodium hydroxide

 iv. No other product

Solution 12

a.  

(ii) No net energy change is involved.

b.  

(iii) Exchange of ions of AB and CD

c.  

(iv) Double displacement reaction

d.  

(i) Carbon dioxide

Chapter 2 - Chemical Changes and Reactions Exercise Ex. 2(C)

Question 1

What is a chemical change? Give two examples of chemical change?

Solution 1

A chemical change is a permanent change in which the chemical composition of a substance is changed and a new substance is formed.

Examples:

Heating of copper carbonate

Formation of curd from milk

Question 2

Why energy is involved in a chemical change?

Solution 2

In every chemical change, change in energy is involved.

There is a difference between the chemical energies of the reactants and products. It involves the breaking up of chemical bonds between the atoms resulting in the absorption of energy in the form of heat and simultaneous formation of bonds with the release of energy.

Question 3

What do you understand by 'chemical reaction'?

Solution 3

A chemical reaction is the process of breaking the chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products).

A chemical change or chemical reaction occurs when particles collide. Collisions occur when reactants are in close contact or by supply of energy.

Question 4

Give an example of a reaction where the following are involved

(a) Evolution of heat

(b) Absorption of heat

(c) High pressure is required

Solution 4

(a) C + O2 → CO2 + Heat

(b) C + 2S   CS2

(c) N2 + 3H2 2NH3

Question 6

Give an example of each of the following chemical changes.

(a) A reaction involving

     (i) Change of state

     (ii) formation of precipitate

(b) An exothermic and an endothermic reaction involving carbon as one of the reactants.

(c) A reaction where colour change is noticed.

Solution 6

(a)

    (i) Change of state

    Ammonia gas reacts with HCl gas to give solid ammonium chloride.

    NH3(g) + HCl(g) ⇋ NH4Cl(s)

   (ii) Formation of precipitate

  When a solution of silver nitrate is added to a solution of sodium chloride, a white insoluble substance, silver chloride, is formed.

   AgNO3(aq) + NaCl(aq)  AgCl(aq)  + NaNO3(aq)

(b)

     Exothermic reaction:

     When carbon burns in oxygen to form carbon dioxide, a lot of heat is produced.

      C + O2 → CO2 + Heat

 

   Endothermic reaction:

   When carbon is heated with sulphur at high temperature, liquid carbon disulphide is formed.

   C + 2S  CS2

(c) Colour change

A few pieces of iron are added into a blue coloured copper sulphate solution; the blue colour of copper sulphate fades and eventually turns into light green due to the formation of ferrous sulphate.

Fe + CuSO4  FeSO4 + Cu 

 

Question 7

Define exothermic and endothermic changes. Give two examples in each case.

Solution 7

Exothermic reaction:

A chemical reaction in which heat is given out is called an exothermic reaction.

Example:

When carbon burns in oxygen to form carbon dioxide, a lot of heat is produced.

C + O2 → CO2 + Heat

When hydrogen is burnt in oxygen, water is formed and heat is released.

2H2 + O2  2H2O + Heat

Endothermic reaction:

A reaction in which heat is absorbed is called endothermic reaction.

Example:

When carbon is heated with sulphur at high temperature, liquid carbon disulphide is formed.

C + 2S   CS2

When nitrogen and oxygen are heated together to a temperature of about 3000°C, nitric oxide gas is formed.

N2 + O2  2NO 

Question 8

State the effects of endothermic and exothermic reactions on the surroundings.

Solution 8

Exothermic reactions are spontaneous and warm their surroundings with the release of heat energy.

Endothermic reactions absorb heat energy from their surroundings and cause their surroundings to cool down.

Question 9

Define:

(a) Photochemical reaction

(b) Electrochemical reaction

Give one example in each case.

Solution 9

(a) It is a reaction which occurs with absorption of light energy.

Example: Photosynthesis

  

(b) It is a reaction which occurs with absorption of electrical energy.

Example:

Acidulated water breaks into hydrogen and oxygen.

  

Question 10

Complete and balance the following reactions:

(a) NaCl(aq) + AgNO3(aq)

(b) Pb(NO3)2 + KI →

(c) CuCO3  

(d) Pb(NO3)2  

(e) NH3 + O2  

Solution 10

(a) NaCl(aq) + AgNO3(aq)AgCl(aq) + NaNO3(aq)

(b) Pb(NO3)2 + 2KI →2KNO3 + PbI2

(c) CuCO3 CuO(s) + CO2(g)

(d) 2Pb(NO3)2 2PbO + 4NO2 + O2

(e) 4NH3 + 5O2 4NO +6H2O 

Question 11

What do you observe? When

  1. Lead nitrate is heated.
  2. Silver chloride is exposed to sunlight.
  3. Hydrogen peroxide is exposed to sunlight.
  4. H2S gas is passed through copper sulphate solution.
  5. Barium chloride is added to sodium sulphate solution
  6. Water is added to the quick lime.
  7. Sodium chloride solution is added to silver nitrate solution.
Solution 11
  1. Lead nitrate decomposes on heating, leaving a yellow residue of lead monoxide, and brown nitrogen dioxide and colourless oxygen gases are evolved.
  2. Due to thermal decomposition, silver chloride breaks down into silver and chloride.
  3. Hydrogen peroxide breaks down to form water and oxygen gas along with heat energy.
  4. When hydrogen sulphide is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is obtained, and sulphuric acid so formed remains in the solution.
  5. A white insoluble precipitate of barium sulphate is formed.
  6. Quick lime reacts vigorously with water to produce slaked lime, i.e. calcium hydroxide.
  7. When sodium chloride is added to the silver nitrate solution, a white curdy precipitate of silver chloride is formed.

 

Question 12

(a) a carbonate which do not decompose on heating.

(b) a nitrate which produces oxygen as the only gas.

(c) a compound which produces carbon dioxide on heating

(d) a nitrate which produces brown gas on heating.

Solution 12

(a) Sodium carbonate

(b) Sodium nitrate

(c) Zinc carbonate

(d) Lead nitrate