Question 1. Define:

1. 1. Inter molecular force of attraction.
   2.  Inter molecular space. 

Answer: (a) The molecules of matter are always in motion and attract each other with a force called inter molecular force of attraction due to which they are held together.

(b) The molecules can move only when there are gaps or space between them, this space is called inter molecular space. 

Question 2. Why do solids, liquids and gases differ in their physical states ? Answer: 

1. • Inter molecular force of attraction.
   • Inter molecular spaces are two important properties of matter that account for the different states of matter. 

Question 3. What are fluids ? Give two examples.

Answer: Substances that can flow are called fluids, e.g. gases (oxygen, hydrogen), liquids (water, petrol, sulphuric acid). 

Question 4. Define inter conversion of states of matter.

Answer: The process by which matter changes from one state to another and back to original state, without any change in its chemical composition. 

Question 5. What are the two conditions for the inter conversion of states of matter ?

Answer: Two conditions are : change in 1. Temperature 2. Pressure 

Question 6. How a liquid changes into its gaseous state ? Explain ?

Answer: As a liquid is heated, its particles starts gaining energy and move more vigorously which 

increases the gaps between the particles and decreasing the force of attraction. Ultimately a liquid changes into gaseous state. 

Question 7. Water cycle is an example of inter conversion of states of water. Explain.

Answer: Water from oceans, rivers lakes from leaves of trees (transpiration) changes into vapors when temperature increases or evaporates and enters the atmosphere as clouds when temperature falls the vapors change into water and some of it in the form of snow fall on mountains and earth in the form of water and hales and this continues. Thus water cycle is example of inter conversion of states of water. 

Question 8. State the general properties of a solid.

Answer: General Properties Of A Solid : 

• • Solids are hard that is, they have a definite shape and volume.
  • Solids are generally in compressible.
  • Solids are rigid, that is, they do not flow. 

Question 9. What is the relation between inter molecular space and inter molecular force ?

Answer: The force of attraction between the molecules of a given substance is called inter molecular force and the space between these molecules is called inter molecular space. The basic relation between the two is that they are inversely proportional to each other. More is the inter molecular force lesser is the inter molecular space and vice-versa. 

Question 10. Why liquids do not have a definite shape ?

Answer: Molecules of a liquid are held by weak inter molecular forces. This force is strong enough to hold the molecules together but not strong enough to hold them at fixed positions. As a result liquids have a fixed volume but not shape. 

Question 11. What happens when a solid is heated ?

Answer: When a solid is heated, its molecules gain energy and vibrate faster. A stage comes when they overcome inter molecular force of attraction and start moving from each other. This results in melting of solid. 

Question 12. Give reasons for the following. 

1. Gases can be compressed easily : The reason for this property of gases is that there is very large inter molecular space between gas molecules. On mere applying pressure, they are easily compressed.

2. Liquids can flow easily : In liquids intermolecular force is weaker than that of solids. So molecules in a liquids can slip over one another and liquids can flow unlike solids.

3. We need to classify things : We need to classify things in order to distinguish them. In this way, things can be categorized and can be easily studied.

4. Pure substances have fixed melting or boiling point : Pure substances consists of only one kind of matter. All the particles of a pure substance are alike. It has a definite composition and similar properties. This is the reason that pure substances have fixed melting or boiling points.

5. Electricity is not considered matter : Electricity neither has mass nor it occupies space. Beside it can not be seen by our eyes. This is why electricity is not considered matter. 

Question 13. Define the following terms. 

1. Matter—Anything that has mass and occupies space is called matter.
2. Inter molecular force — The force of attraction between the molecules of a given substance is called inter molecular force.
3. Element — It is defined as that pure substance which contains only one type of atoms e.g. hydrogen, chlorine.
4. Atom — An atom is the smallest part of an element that takes place in a chemical reaction.
5. Molecule — A molecule is a smallest part of a compound that exists independently. 

Question 14. Write your observation and conclusion for the following: 

1. When a small stone is gently dipped into a glass filled with water.
2. When one of the balloons suspended to the metre scale is punctured while other remains inflated ? 

Answer: 

1. You will see that some water flows out of the tumbler and collects in the bowl. Remove the stone from the tumble. The level of water in the tumbler comes down. Now, pour the water collected in the bowl back into the tumbler. The glass tumbler is filled again. This is because the stone occupied space and therefore drives the water out of the tumbler. This proves that not only solids but liquids also occupy space.
2. Take two similar balloons and inflate them equally. Suspend one balloon to the left of a metre scale and the other one to the right of it, as shown in the figure below. Balance the scale in the middle with the help of a peg. 

Question 15. How will you prove by an experiment that solids occupy space ? Answer: Experiment to show that solids occupy space : Take a glass bowl. Place a glass tumbler full of water in it. Now tie a stone with thread and lower into the water. Some water flows out of tumbler into bowl. When we remove stone from tumbler the level of water in tumbler comes down. This shows that solids occupy space. 

Question 16. Name the three states of matter and define them.

Answer: There are three states of matter 

• Solid State : The molecules are very close to each other hence inter molecular spaces are small and inter molecular force is strong. Hence solids have definite volume, rigid, retain definite shape and are in compressible. 
• Liquids : The molecules are less closely packed have more inter molecular spaces than solid, less stronger forces than solids. Hence liquids have definite volume but no definite shape. They take the shape of container in which they are put. 
• Gases: The molecules in the gases are for apart with weakest force of attraction. Hence gases have neither definite volume nor definite shape and compressible. 

Question 17. Give reasons : 

1. Why do liquids and gases flow but solids do not ? 

The molecules of liquids and gases are far apart i.e. have more gaps, inter molecular attraction force is very less as compared to solids, hence liquids and gases can flow but solids do not as gaps in solid molecules is less and molecular force of attraction very strong.

2. A gas fills up the space available to it. 

Inter molecular force of attraction is least and inter molecular spaces are very large, hence gases can fill up the space available to them.

3. The odour of scent spreads in a room. 

Scent fumes (molecules) being gases fill the spaces between air molecules and the molecules of air fill the spaces between scent molecules due to diffusion, fumes spread into a room. OR Due to inter-mixing of scent molecules and air molecules, scent fumes spread into the room.

4. We can walk through air. 

The molecules of air are far apart i.e. large gaps and we can walk through air easily. 

5. Liquids have definite volume but no definite shape. 

The molecules of liquid are loosely packed and inter molecular force of attraction is small but number of molecules in it remain the same. Hence liquids have definite volume but no definite shape. 

Question 18. Give reasons :

(a) When a teaspoon of sugar is added to half a glass of water and stirred, the water level in the glass does not rise. Add one teaspoon of sugar to it and stir. The sugar disappear but the level of water in the glass does not rise that means the volume of water has not increased. Because the sugar particles are adjusted between the water molecules. The shows that there are inter molecular gaps in water. 

(b) When an empty gas jar is inverted over a gas jar containing a colored gas, the gas also spreads into the empty jar. This shows that gases can fill up all the space that they get, and they have neither a fixed shape nor a fixed volume. They have no free surfaces, either. 

(c) A red ink drop added to small amount of water in a glass turns water red in some time. If we put a drop of red ink in a glass of water, its particles diffuse with particles of water slowly but continuously and the water turns red. 

Question 19. Give an experiment to explain that there are inter molecular spaces between water molecules.

Answer: Take a completely filled glass of water. Add a spoon full of sugar. Stir it well. The volume of water hence the level of water in glass remains the same where has gone the volume of sugar added? Actually the sugar molecules took the spaces (gaps) between the molecules of water and level of water in glass remains the same. This shows that there are inter molecular spaces between the water molecules. 

Question 20. Differentiate between the following.

(a) Atoms and Molecules Atoms Molecules 

• • Atom: It is the smallest part of an element.
  • Molecule: It is the smallest part of a compound. 
  • Atom: It does not have independent existence.
  • Molecule: It has an independent existence