ICSE Living Science Chemistry for Class 8 Solutions Chapter 1 Matter

Think And Answer (Page 4)

Question 1.
When water is cooled, it changes to ice. Why?
Answer:
When the temperature is decreased, the molecules of a water lose kinetic energy gradually and slow down. As a result, the intermolecular force of attraction between them increases and ‘ the molecules come close together. With continued cooling, the intermolecular space decreases to such an extent that water changes to a solid ice.

Question 2.
A gas completely fills the container in which it is kept. Why?
Answer:
The intermolecular force of attraction is the weakest in gases. , So, the intermolecular space in gases is the largest in comparison to liquids and solids. As a result, the molecules in gases are very far apart and randomly move about in the whole space available to them.

Check Your Progress (Page 7)

Write true or false. Correct the false statements
Question 1.
Molecules in motion possess energy called kinetic energy.
Answer:
True.

Question 2.
The filled-up spaces between the different molecules are called the intermolecular spaces.
Answer:
True.

Question 3.
The force of attraction between molecules of a substance is called the intermolecular force of attraction.
Answer:
True.

Question 4.
The intermolecular force of attraction in liquids is as strong as that in solids.
Answer:
False. The intermolecular force of attraction in solids is stronger as that in liquids.

Question 5.
The molecules in gases frequently collide with each other and the sides of the container in which they are kept.
Answer:
True.

A. Tick the most appropriate answer.

1. In which of the following is the intermolecular space between molecules the least?
a. solids
b. liquids
c. gases
d. none of these
Answer:
a. solids

Question 2.
What happens to the intermolecular force of attraction as the intermolecular space between molecules decreases?
a. It decreases.
b. It increases.
c. It does not change.
d. It may increase or decrease.
Answer:
b. It increases.

Question 3.
In which of the following is the intermolecular force the weakest?
a. solids
b. liquids
c. gases
d. none of these
Answer:
c. gases

Question 4.
Which form of energy do the molecules of a substance gain when it is heated?
a. electrical
b. chemical
c. water
d. kinetic
Answer:
d. kinetic

Question 5.
Which of the following statements is false with respect to the law of conservation of mass?
a. Matter is neither created nor destroyed during a chemical reaction.
b. There is no change is mass during a chemical reaction.
c. The mass of the products equals the mass of the reactants in a chemical reaction.
d. There is a change in mass during a chemical reaction,
Answer:
d. There is a change in mass during a chemical reaction.

B. Fill in the blanks.

Question 1.
The empty spaces between different molecules are called ……….. or ……….. spaces.
Answer:
The empty spaces between different molecules are called inter-particle or intermolecular spaces.

Question 2.
The force of attraction between molecules of the same kind is called ………..
Answer:
The force of attraction between molecules of the same kind is called cohesive.

Question 3.
The force of attraction between molecules of different kinds is. called ………..
Answer:
The force of attraction between molecules of different kinds is called adhesive.

Question 4.
The collisions between the molecules and container exert ……….. on the walls of the container.
Answer:
The collisions between the molecules and container exert pressure on the walls of the container.

Question 5.
When a solid is heated, its molecules gain ……….. energy.
Answer:
When a solid is heated, its molecules gain kinetic energy.

Question 6.
Matter is neither ……….. nor ……….. during a chemical reaction.
Answer:
Matter is neither created nor destroyed during a chemical reaction.

C. Write true or false. Correct the false statements.

Question 1.
The molecules of a substance collide with each other.
Answer:
True

Question 2.
In solids, molecules move randomly.
Answer:
False; In gases molecules move randomly.

Question 3.
As the temperature of a substance increases, its molecules move faster and thus the kinetic energy of the molecules increases.
Answer:
True

Question 4.
The intermolecular force of attraction in liquids is not as strong as that in solids.
Answer:
True

Question 5.
The intermolecular force of attraction is the strongest in gases.
Answer:
False; The intermolecular force of attraction is the strongest in solids.

Question 6.
When a solid is heated, its molecules lose kinetic energy and become very active.
Answer:
False; When a solid is heated, its molecules gain kinetic energy and become very active.

D. Match the columns.

ICSE Living Chemistry for Class 8 Solutions Chapter 1 Matter - 1
Answer:
ICSE Living Chemistry for Class 8 Solutions Chapter 1 Matter - 2

E. Name the following.

Question 1.
The energy possessed by molecules in motion.
Answer:
Kinetic energy

Question 2.
The theory that enables us to understand the behaviour of molecules of solids, liquids and gases.
Answer:
Kinetic theory of matter

Question 3.
The empty spaces between different molecules.
Answer:
Inter-particle or intermolecular space

Question 4.
The attractive forces between the molecules of a substance.
Answer:
Inter-particle or intermolecular force of attraction

Question 5.
The law that states that the mass of the products equals the mass of the reactants in a chemical reaction.
Answer:
Law of conservation of mass

F. Answer the following in short.

Question 1.
What are the empty spaces between the different molecules of matter called?
Answer:
The empty spaces between different molecules are called interparticle or intermolecular spaces.

Question 2.
What is the force of attraction between the molecules of a substance called?
Answer:
The force of attraction between the molecules is called inter¬particle or intermolecular force of attraction.

Question 3.
What is cohesive force?
Answer:
The force of attraction between the molecules of the same kind is called cohesive force.

Question 4.
What is adhesive force?
Answer:
The force of attraction between the molecules of different kinds is called adhesive force.

Question 5.
When a solid is heated, what happens to its molecules?
Answer:
When a solid is heated, its molecules gain kinetic energy and become very active.

Question 6.
When a gas is cooled, what happens to its molecules?
Answer:
Similarly, when the temperature is decreased, the molecules of a substance lose kinetic energy gradually and slow down.

G. Answer the following in detail.

Question 1.
List the main postulates of the kinetic molecular theory of matter.
Answer:
The main postulates of the kinetic molecular theory of matter are as follows.
1. Composition of matter: Matter is composed of molecules. The molecules of a given substance are identical in all respects.

2. Movement of molecules and kinetic energy : Molecules in matter move constantly. Due to this movement, molecules possess energy called kinetic energy.

3. Intermolecular space : There are empty spaces between the molecules of matter. The empty spaces between different molecules are called inter-particle or intermolecular spaces. The intermolecular space between the molecules in gases is the largest.

4. Intermolecular force of attraction : There are attractive forces between the molecules of a substance. The force of attraction between molecules is called inter-particle or intermolecular force of attraction. The intermolecular force of attraction between , the molecules in solids is the largest

5. Collision of molecules : The molecules of a substance collide with each other. In the gaseous and liquid states, the molecules also collide with the walls of the container in which the substance is kept. The collisions between the molecules and the container exert pressure on the walls of the container.

6. Movement of molecules in different states : The molecules of matter are in motion. In solids, they only vibrate. In liquids and gases, they move randomly and thus possess kinetic eneigy.

Question 2.
Differentiate between cohesive force and adhesive force.
Answer:
The force of attraction between the molecules of the same kind is called cohesive force. The force of attraction between the molecules of different kinds is called adhesive force.

Question 3.
Explain the nature of molecules in solids, liquids and gases.
Answer:
The molecules in solids hardly move about; they only vibrate about their positions. The molecules in liquids collide more frequently with each other than in gases due to shorter distances between them. The molecules in gases also frequently collide with each other and with the sides of the container in which they are kept.

Question 4.
Explain the change of state of matter on cooling.
Answer:
Similarly, when the temperature is decreased, the molecules of a substance lose kinetic energy gradually and slow down. As a result, the intermolecular force of attraction between them increases, and the molecules come close together. With continued cooling, the intermolecular space decreases to such an extent that a gas changes to a liquid and a liquid changes to a solid.

Question 5.
Explain the law of conservation of mass with an example.
Answer:
The law of conservation of mass states that matter is neither created nor destroyed during a chemical reaction. In a chemical reaction, a change in the composition of substances occurs, but there is no overall change in mass.
For example, as magnesium burns in oxygen to produce magnesium oxide, the total mass of the oxygen and magnesium reacted, equals the mass of the magnesium oxide produced.
magnesium + oxygen → magnesium oxide
mass before the reaction = mass after the reaction

H. Complete these word equations.

1. magnesium + …………. → magnesium oxide
Answer:
magnesium + oxygen → magnesium oxide

2. …………. + hydrogen ⇌ ammonia
Answer:
nitrogen + hydrogen ⇌ ammonia

3. barium chloride + sodium sulphate → ……….. + ………..
Answer:
barium chloride + sodium sulphate → barium sulphate (white precipitate) + sodium chloride

Living Science Chemistry Class 8 ICSE Solutions

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