1. Water can change from solid to liquid to gas and back to liquid and solid. Explain.

Water moves between the oceans, atmosphere and land in a huge circular path called water cycle. Water turns into a gas called water vapour by evaporation at any water surface. In the cool air, the water vapour condense to form millions of water droplets that make the clouds. At the top of the clouds, it is so cold that the droplets freeze and form snowflakes. They fall through the cloud and melt to form raindrops. Falling water in the form of rain, snow or hot is called precipitation.

2. How are the particles arranged in the three states of matter?

In solids, strong forces hold the particles together in a three - dimensional structure. The particles do not change position, but vibrate to and fro about one position. In liquids, the forces that hold the particles together are weaker than in solids. They can change position by sliding over each other. In gases, the forces of attraction between the particles are very small and the particles can move away from each other and travel in all directions. When they hit each other or the surface of their container they bounce and change the direction.

3. Define freezing.

If a liquid in cooled sufficiently, the particles lose so much energy that they can no longer slide over each other. The only movement possible is the vibration to and fro about one position in the lattice. The liquid has became a solid. This is called freezing.

4. What is meant by evaporation?

The particles in a liquid have different amounts of energy. The particles with the most energy move the fastest. High energy liquid particles near the surface move so fast that they can break through the surface and escape into the air and form a gas. This is called evaporation.

5. Give the changes that takes place during boiling.

When a liquid is heated, all the particles receive more energy and move quickly. The fastest moving particles escape from the liquid surface or collect in the liquid to form bubbles. The bubbles rise to the surface and burst open into the air. The fast moving particles released from the liquid form a gas.

6. How is boiling different from sublimation?

When a few substances such as iodine is heated, the energy the particles receive makes them separate and form a gas without forming a liquid first. This is called sublimation. In boiling, the particles receive more energy and more quickly they escape from liquid surface and form bubbles. The fast moving particles released from the liquid form a gas.

7. What is meant by pressure?

A gas does not have a surface like a solid or a liquid, but it still pushes on any surface with which it makes contact. This push on the surface area of a liquid or solid is called pressure. The quickly moving particles are burning off the walls of the gas container. The force of these particles as they push against the surface gives rise to the gas pressure.

8. Define condensation.

The particles in a gas possess a large amount of energy which they use to move. If the particles are cooled, they lose some of their energy and slow down. If the gas is cooled sufficiently, the particles lose so much energy that they can no longer bounce off each other when they meet. The particles now slide over each other and form a liquid.

9. Conditions on other planet in the solar system are very different from that on Earth. Explain.

Jupiter is made from a very large amount of hydrogen. The pressure and temperature near the centre of the planet have made the hydrogen there into a solid like a metal. Above the solid hydrogen there is a vast ocean of liquid hydrogen. Uranus is made from ammonia, methane and water. Earth is the only planet with water and having living forms in it.

10. Give an account for the atmospheric pressure.

The atmosphere is a mixture of gases that covers the surface of the Earth. The atmosphere is 1000 km thick and pushes on every square centimetre of the Earth’s surface. The pressure of the atmosphere at sea level is called standard pressure and is about 10N/Cm².  It is the pressure at which the boiling point of any substance is measured. At the top of very high mountains, the pressure of the atmosphere is less than at sea level.