Class 8 General Science Notes: - Unit-6: Chemical Reaction and Bonding: Solved Exercise (Short Questions and Answers)

(National Book Foundation - New Book/New Syllabus Based on National Curriculum Pakistan 2023-2024 and Onwards)

(Class 8 General Science Notes for 8th Centralized Exam 2023-2024 and onward, FDE, Islamabad)

B. Short questions

i). What is a chemical reaction? Give two samples

Ans:

A chemical change is called a chemical reaction

The conversion of a substance into a new substance is called a chemical reaction

Example:

1. Burning of coal or natural gas

2. Rusting of iron

ii) State the law of conservation of mass.

Ans:

It states that the total mass of all the products of a chemical reaction is equal to the total mass of the reactants.

iii) Write a balanced chemical equation for the reaction between methane and oxygen.

Ans:

iv) Show the formation of NaCl by electron-dot and electron-cross structures.

Ans:

(page 96)

v). Show the formation of MgO by electron-dot and electron-cross structures (Atomic number Mg=12, O=8)

Ans:

v) Define the following
a) Double covalent bond
b) Triple covalent bond

Ans:

Double Covalent Bond: The sharing of pairs of electrons between two atoms is called Double Covalent Bond.

Example: Formation of Oxygen molecule O₂

Triple Covalent Bond: Triple covalent bond is formed by sharing of three pairs of electrons between two atoms is called Triple Covalent Bond.

Example: Formation of Nitrogen molecule N

vii). What is a double displacement reaction? Give one example.

Ans:

A chemical reaction in which two atoms or groups of atoms exchange places and form new compounds is called a double displacement reaction.

Example:

A reaction between an acid (HCl) and a base (NaOH) produces new compounds.

NaOH + HCl ⟶ NaCl + H₂O

viii). Give one example for each of the following reactions.
a) combustion b) combination c)Displacement

Ans:

a) Combustion: Burning of methane (Sui gas) is a combustion reaction.

b) Combination: During electrical discharge in the atmosphere nitrogen combines with oxygen to produce nitric oxide (NO)

c) Displacement: Iron can be extracted from iron oxide (Fe2O3) by heating with coke (carbon).

ix) Give one example of each of the following from daily life.
a) Exothermic reaction b) Endothermic reaction

Ans:

a) Exothermic reaction:

An exothermic reaction takes place in our bodies in every cell. During respiration, glucose molecule combines with oxygen and breaks down

into carbon dioxide and water and release energy.

b) Endothermic reaction:

When cake dough is heated in an oven, an endothermic reaction occurs. Heat absorbed changes cake dough into a fluffy and delicious material.

x) Show the formation of a covalent bond between hydrogen and chlorine.

Ans:

Unit-6: Chemical Reaction and Bonding: Solved Long Questions and Answers

Class 8 General Science Notes

Source: General Science Book (National Book Foundation, Pakistan)

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