1. Short/Long answer questions 

Question 1. Define matter. What is its composition ?

Answer: Matter is defined as anything which occupies space and has mass. It can be perceived by our sense of smell, touch, sight, hearing and taste. Matter is composed of tiny particles known as atoms. 

Question 2. Name the three states of matter.

Answer: The three states of matter are solids, liquids and gases.

Solids —A solid has a definite shape and definite volume. Example – wood, stone, iron, ice etc. Liquid —A liquid has a definite volume but not definite shape. Example — water, juice, milk, oil, etc. Gases —A gas neither has definite shape nor a definite volume. Example – air, hydrogen, oxygen, watervapour etc. 

Question 3. What is a molecule ?

Answer: The smallest unit of matter which can exist independently is called molecule. Example: Oxygen molecule (O2) made up of two (O) atoms. 

Question 4. What is the approximate size of a molecule ?

Answer: Matter is made up of molecules which are very small in size (~10-9 m). 

Question 5. One litre of water has 6.02 × 1026 molecules. Estimate the size of a molecule.

Answer: The size of a particle (or molecule of matter is very small. 1 litre of water has 6.02 × 1026 molecules, so the volume of a particle of 

Thus the diameter of a water molecule is nearly 1.27 × 10-9 metre. 

Question 6. What do you mean by inter-molecular spacing ?

Answer: Inter molecular space — The space between any two consecutive molecules of a substance is called inter molecular space 

Question 7. Describe a simple experiment to illustrate the existence of inter-molecular spacing.

Answer: Take 100 ml of water in a measuring cylinder. Add 20 gram of salt in water gently and stir it well so as to dissolve the salt well in water. It is noticed that the level of water does not change. It shows that the particles of salt occupy spaces between the particles of water. 

Question 8. What do you mean by inter-molecular forces ?

Answer: Inter molecular force of attraction — The force of attraction between the molecules (like molecules or unlike molecules) is called inter molecular force of attraction. 

Question 9. What are the forces of cohesion and adhesion ?

Answer: The force of attraction between the molecules of similar kind is called force of cohesion.

Example: The forces between water molecules. This force of cohesion keep the molecules of the substance bind together. The force of attraction between different types of molecules is called force of adhesion.

Example: When a glass filled with water is emptied some water I particles remain stuck to the glass due to the adhesion between water molecules and glass. 

Question 10. State three characteristics of molecules of matter.

Answer: The particles of matter called molecules, have the following characteristics: 

1. • They are very small in size.
   • They have spaces between them.
   • They are in constant random motion.
   • They always attract each other. 

Question 11. The molecules in a substance are in motion. What type of path do they follow ?

Answer: The particles in a substance are not at rest (in motion),and they move randomly in all possible directions in a zig-zag Path 

Question 12. Describe a simple experiment to illustrate that molecules are not at rest, but they constantly move.

Answer: Take a beaker. Fill it partly with water. Add some lycopodium powder in the beaker containing water. Stir the contents of the beaker with a glass rod. Take out few drops of this suspension on a glass plate. Place it on the table and illuminate it with a table lamp. Observe the glass plate through a microscope. It is found that the fine particles of lycopodium powder move rapidly in a random manner and their path is zig zag as shown in figure below. 

Question 13. Write down five general properties of solids, liquids and gases. Answer:

Solids: 

1. • The molecules here are very tightly packed having negligible or very less intermolecular space.
   • They have the strongest intermolecular force of attraction.
   • The molecules have very small vibration about their mean position i.e. small amplitude.
   • They have a definite shape and volume.
   • They are generally hard and rigid.
   • They are good conductors of heat. 

Liquids: 

1. • Molecules are less tightly packed.
   • The intermolecular force of attraction is less than that of solids.
   • The molecules here can move from one place to another
   • Do not have any particular shape of their own and thus acquire the shape of the vessel.
   • A particular quantity of a liquid has a definite volume at a given temperature. 

Gases : 

1. • The force of attraction between the molecules is the least.
   • The inter molecular space is the largest.
   • Neither have a definite shape nor a definite volume.
   • The molecules move independently.
   • Worst conductors of heat. 

Question 14. Give the molecular model for a solid and use it to explain why a solid has a definite volume and a definite shape.

Answer: Here the molecules are very tightly packed that there is no or very less inter molecular space and there is high inter molecular force of attraction (force of cohesion). The molecules do not move about their mean position and thus solids have a definite shape and volume. 

Question 15. Describe the molecular model for a liquid. I-low does it explain that a liquid has no definite shape, but has a definite Volume ?

Answer: Here the molecules are less tightly packed as compared to solids and also there is lesser force of inter molecular attraction. The inter molecular distance is greater than that in the solids. Thus, they do not have a definite shape but acquire the shape of the vessel in which they are contained but have a definite volume at a given temperature. 

Question 16. A gas has neither a definite volume nor a definite shape. Describe the molecular model to explain it.

Answer: Here the molecules are far apart from each other i.e. have the greatest inter molecular distance which result into the weakest inter molecular forces of attraction. The molecules as are not bound by any strong force move about freely and thus gases do not have a definite shape and also do not have any definite volume. 

Question 17. Distinguish between the three states of matter—solid, liquid and gas on the basis of their molecular models.

Answer:

Solids: Here the molecules are very tightly packed that there is no or very less inter molecular space and there is high inter molecular force of attraction (force of cohesion). The molecules do not move about their mean position and thus solids have a definite shape and volume.

Liquids: Here the molecules are less tightly packed as compared to solids and also there is lesser force of inter molecular attraction. The inter molecular distance is greater than that in the solids. Thus, they do not have a definite shape but acquire the shape of the vessel in which they are contained but have a definite volume at a given temperature.

Gases : Here the molecules are far apart from each other i.e. have the greatest inter molecular distance which result into the weakest inter molecular forces of attraction. The molecules as are not bound by any strong force move about freely and thus gases do not have a definite shape and also do not have any definite volume. 

Question 18. What do you mean by the change of state of matter ? Explain:

(a) the change of a solid into a liquid at a constant temperature, and

(b) the change of a liquid into a gas at a constant temperature.

Answer: The change in state of matter of a substance from solid to liquid or from liquid to gas is brought by imparting heat energy to it at a constant temperature.

(a) The process of change of a substance from solid state into its liquid state on absorption of heat at a particular temperature, called the melting point, is called melting or fusion i. e. 

(b) The process of change of a substance from a liquid state to its gaseous state at a particular temperature, called the boiling point, is called boiling or vaporisation