CHEMICAL REACTIONS AND THEIR TYPES

The substance taking part in a chemical reaction is

called reactant.

2. Product : The substance formed during the chemical reaction is

called product.

3. Chemical reaction : The reaction in which the products are

formed from the reactants is called a chemical reaction.

4. Chemical equation : A symbolic representation of a chemical

reaction using symbols or formulae of reactants and products is

called chemical equation.

5. The steps of writing a chemical equation : (1) The molecular

formulae of the reactants are written on the left hand side of the

equation. (2) The molecular formulae of the products are written

on the right hand side of the equation. (3) Reactants and

products are connected with an arrow pointing towards the

product side.

6. Types of chemical reactions : (1) Combination reactions

(2) Decomposition reactions (3) Displacement reactions.

7. Combination reaction : The chemical reactions in which a

single compound (product) is formed from two or more reactants

is called a combination reaction.

8. Decomposition reaction : The chemical reaction in which a

substance is broken down into two or more substances is called a

decomposition reaction.

9. Displacement reaction : The chemical reaction in which one

constituent displaces another constituent and separates is

called a displacement reaction.

10. Oxidation : A chemical reaction in which oxygen combines

with an element or a compound or a reaction in which

hydrogen is lost from a compound is called an oxidation

reaction.

11. Reduction : A chemical reaction in which hydrogen combines

with an element or compound or a reaction in which oxygen is

lost from a compound is called a reduction reaction.

12. The speed of chemical reactions : The speed of chemical

reactions depend on the following factors : (1) the size of the

reactant particles (2) the concentration of the reactants (3) the

temperature (4) catalyst used in the chemical reaction.

13. Catalyst : A substance which does not take part in a chemical

reaction but by its mere presence speeds up a chemical

reaction, is called a catalyst.

In the preparation of oxygen from potassium chlorate,

manganese dioxide is used as a catalyst.

14. Exothermic reaction : A reaction in which heat is given out is

called an exothermic reaction.

The reaction between calcium oxide and water is an

example of exothermic reaction.

15. Endothermic reaction : A reaction in which heat is absorbed

is called an endothermic reaction.

The reaction between salt and water is an example of

endothermic reaction.

What are the steps of writing a

 chemical equation?

Ans. (1) The molecular formulae of the reactants are written

on the left hand side of the equation. (2) The molecular formulae of

the products are written on the right hand side of the equation.

(3) Reactant side and product side are connected with an arrow

pointing towards the product side

What is meant by chemical equation ?

Ans. A symbolic representation of a chemical reaction using

symbols or formulae of reactants and products is called chemical

equation.

Name the different type of chemical reactions.

Ans. The different type of chemical reactions are : (1) Combination

reactions (2) Decomposition reactions (3) Displacement

reactions.

Define the following :

(1) Combination reaction (2) Decomposition reaction

(3) Displacement reaction.

Ans. (1) Combination reaction : The chemical reaction in

which a single product (compound) is formed from two or more

reactants is called a combination reaction.

(2) Decomposition reaction : The chemical reaction in which

a substance is broken down into two or more substances is called a

decomposition reaction.

(3) Displacement reaction : The chemical reaction in which

one constituent displaces another constituent and separates, is

called a displacement reaction.

Combination reaction : The reaction between

calcium oxide and water produces a single compound (product),

calcium hydroxide.

Decomposition reaction : When an electric current is

passed through acidulated water, it is decomposed to liberate

hydrogen and oxygen

Displacement reaction : Zinc reacts with dilute

hydrochloric acid to form zinc chloride and hydrogen. In this

reaction, zinc displaces hydrogen from HCl.

Explain with an example what is meant by oxidation.

Ans. A chemical reaction in which oxygen combines with an

element or a compound or a reaction in which hydrogen is lost

from a compound is called an oxidation reaction.

Example : Carbon combines with oxygen to form carbon

dioxide.

Explain with an example what is meant by reduction.

Ans. A chemical reaction in which hydrogen combines with

an element or compound or a reaction in which oxygen is lost from

a compound is called a reduction reaction.

Example : Chlorine combines with hydrogen to form

hydrogen chloride.

On what factors does the speed of chemical reactions

depend?

Ans. The speed of chemical reactions depend on following

factors :

(1) the size of the reactant particles (2) the concentration of

the reactants (3) the temperature (4) catalyst used in the chemical

reaction.

What is a catalyst ?

Ans. A substance which does not take part in a chemical

reaction but by its mere presence speeds up a chemical reaction

is called a catalyst.

Name two catalysts and the reactions in which they

are used.

Ans. (1) Manganese dioxide : In the preparation of oxygen

from potassium chlorate, manganese dioxide is used as a catalyst.

(2) Raney nickel : In the preparation of hydrogenated oil

(vanaspati ghee) from vegetable oil, Raney nickel is used as

catalyst.

(1) Will the mass of the catalyst change during a

chemical reaction ?

Ans. No, the mass of the catalyst remains the same during

a chemical reaction.

* (2) Why do we use powdered sugar when making a

sherbet?

Ans. The particles of powdered sugar are extremely small

and powder dissolves easily in a sherbet. Therefore, while

making a sherbet, powdered sugar is used.

Explain with examples what is meant by an

exothermic reaction.

Ans. A reaction in which heat is given out is called an

exothermic reaction.

Example : Take some water in a test tube. Feel the

temperature of the test tube. Add some calcium oxide and stir the

mixture, a solution of calcium hydroxide is formed. Feel the

temperature of test tube again. You will find that the temperature

70 NAVNEET GENERAL SCIENCE DIGEST : STANDARD VIII

has increased slightly. This shows that the reaction is an

exothermic reaction.

Explain with examples what is meant by an

endothermic reaction.

Ans. A reaction in which heat is absorbed is called an

endothermic reaction.

Example : Take some water in a test tube. Feel the

temperature of the test tube. Add some salt and stir the mixture, a

solution of salt is formed. Feel the temperature of test tube again.

You will find that the temperature has fallen slightly. This shows

that the reaction is an endothermic reaction.

Combination reaction Decomposition reaction

1. In combination reaction, two

or more reactants takes part

in a chemical reaction.

1. In decomposition reaction,

only one reactant takes part

in a chemical reaction.

2. In combination reaction, only

one product is formed.

2. In decomposition reaction,

two or more products are

formed.

(1) How are chemical reactions written ?

Ans. Chemical reactions are written in the form of equations.

(2) What happens during a chemical reaction ?

Ans. The products are formed during a chemical reaction.

(3) What is meant by reactant ?

Ans. The substance taking part in a chemical reaction is called

a reactant.

(4) What is meant by product ?

Ans. The substance formed during the reaction is called a

product.

(5) What is meant by a chemical reaction ?

Ans. The reaction in which the products are formed from the

reactants is called a chemical reaction.

(6) What happens in a combination reaction ?

Ans. A single compound (product) is formed from two or more

reactants during a combination reaction.

(7) What happens in a displacement reaction ?

Ans. In a displacement reaction, one constituent displaces

another constituent and separates.

(8) What happens in a decomposition reaction ?

Ans. A single substance is broken down and two or more

substances are formed during a decomposition reaction.

(9) Name the catalyst used in the preparation of oxygen.

Ans. Manganese dioxide is used as catalyst in the preparation

of oxygen from potassium chlorate.

(10) What happens when sulphur is heated?

Ans. When sulphur is heated, it burns in air and forms

sulphur dioxide.

(11) What happens when copper sulphate is heated ?

Ans. When copper sulphate is heated, blue coloured copper

sulphate changes to colourless copper sulphate.

Q. 6 State whether the following statements are True or

False. If a statement is false, correct it and rewrite :

(1) The reaction between calcium oxide and water is an

example of a combination reaction.

(2) The reaction between salt and water is an example of an

exothermic reaction.

(3) Rusting of iron is a fast reaction.

(4) When washing soda is mixed with lime juice, carbon

dioxide bubbles out. This reaction is a quick reaction.

(5) A catalyst speeds up the chemical reaction.

(6) On heating, the colour of copper sulphate remain blue.

(7) When hydrogen combines with chlorine, hydrogen chloride

is formed. This reaction is an example of a reduction

reaction.

(8) Calcium hydroxide is formed in a chemical reaction. Hence,

it is the reactant.

Ans. (1) True. (2) False. The reaction between salt and water

is an example of an endothermic reaction. (3) False. Rusting of iron

is a slow reaction. (4) True. (5) True. (6) False. On heating, copper

sulphate becomes colourless. (7) True. (8) False. Calcium

hydroxide is a product.

Q. 7 Fill in the blanks :

(1) Zinc reacts with hydrochloric acid. The reaction is ………

reaction.

(2) ……… is used in the hydrogenation of vegetable oil.

(3) The substances formed during the reaction are called

……….

(4) A reaction in which heat is given out is called an ………

reaction.

(5) The process of rusting of iron is a ……… process.

Ans. (1) a displacement (2) Raney nickel (3) products

(4) exothermic (5) slow.

Q. 8 Choose the correct options and complete the

following statements :

(1) The reaction between carbon and oxygen is………reaction.

(a) a decomposition (b) a reduction

(c) an oxidation (d) a displacement

(2) The reaction between washing soda and lime juice is ………

reaction.

(a) a slow (b) a quick

(c) an endothermic (d) a displacement

(3) CaO_H

_

O IIIIIIIIJ Ca (OH)

_

. This reaction is ………

reaction.

(a) a decomposition (b) a combination

(c) an endothermic (d) a displacement

(4) When quick lime is added to water, ……… is formed.

(a) calcium oxide (b) calcium

(c) calcium carbonate (d) calcium hydroxide

(5) The reaction between hydrogen and chlorine is ………

reaction.

(a) a decomposition (b) a displacement

(c) a reduction (d) an oxidation

Ans. (1) an oxidation (2) a quick (3) a combination

(4) Calcium hydroxide (5) a reduction.

Q. 9 Match the following :

* [1] ‘A’ ‘B’

(1) Reduction

(2) Oxidation

(3) Displacement

(4) Catalyst

(a) Type of chemical reaction

(b) Combining with hydrogen

(c) Speeding up a reaction

(d) Combination with oxygen

Ans. (1) Reduction – Combining with hydrogen (2) Oxidation

– Combination with oxygen (3) Displacement– Type of

chemical reaction (4) Catalyst – Speeding up a reaction.

[2] ‘A’ ‘B’

(1) Preparation of ghee from

vegetable oil

(2) Reaction between calcium

oxide and water

(3) Copper sulphate is heated

(4) Milk changes to yoghurt

(a) Exothermic reaction

(b) Colourless product

(c) Slow reaction

(d) Raney nickel

Ans. (1) Preparation of ghee from vegetable oil – Raney nickel

(2) Reaction between calcium oxide and water – Exothermic

reaction (3) Copper sulphate is heated – Colourless product

(4) Milk changes to yoghurt – Slow reaction.