Science Answer the questions.[Che-3M]

When a solid is heated, the heat energy makes its particles vibrate more vigorously. At the melting point, the particles of solid have sufficient energy to overcome the strong forces of attraction holding them in fixed positions and break to form small groups of particles. This heat energy is kinetic energy.

The heat energy supplied to ice during the change of state (at its melting point) is all used up in overcoming (or breaking) the force of attraction between its particles without increasing its kinetic energy. Since the heat (or latent heat) supplied during the change of state does not increase the kinetic energy of the ice cubes, therefore no rise in temperature takes place. The temperature remains constant.

The heat energy supplied to water during the change of state (at its boiling point) is all used up in overcoming (or breaking) the force of attraction between its particles without increasing its kinetic energy. Since the heat (or latent heat) supplied during the change of state does not increase the kinetic energy of the water, therefore no rise in temperature takes place. The temperature remains constant.

Acetone (or perfume) is volatile in nature. When we apply it to our palm, we feel cold. This happens due to the fact that to change from liquid to the vapour state, acetone requires latent heat of vaporization. Acetone takes this latent heat of vaporization from the hand due to which the palm loses heat and feels cold.

The cooling in a desert room cooler is caused by the evaporation of water. The higher temperature on a hot day increases the rate of evaporation of water, and the dryness of air also increases the rate of evaporation of water. And due to this increased rate of evaporation of water, a desert room cooler works better on a hot and dry day.

Both bromine gas and air is made up of tiny moving particles. When a gas jar containing air is inverted over gas jar containing bromine vapour, both bromine and air molecules move and collide with one another and bounce about in all directions due to which we see a uniform red brown colour in both the jars.

We should wear cotton clothes in hot summer days because we perspire more through the pores of the skin during such days. Since, sweat is mainly water and cotton clothes are good absorber of water, they absorb the sweat quickly and expose it to the atmosphere for evaporation. The evaporation of sweat from the cotton clothes takes the latent heat of vaporization from our skin hence the skin loses heat and makes us feel cool and comfortable.

1. The physical states are:

• A-liquid.
• B-gas.
• C-solid.
• D-plasma.
• E-Bose-Einstein condensate (BEC).

2. Ammonium chloride; and the name of the process is sublimation.
3. Carbon dioxide.
4. Water.
5. Plasma is the state of matter that allows the sun and the other stars to glow.

1. Characteristics of a solid:

• Solids have a fixed shape and fixed volume.
• Solids do not flow.

2. Characteristics of a liquid:

• Liquids have a fixed volume but no fixed shape, they take the shape of the vessel in which they are placed.
• They generally flow easily.

3. Characteristics of a gas:

• Gases can be compressed easily.
• Gases fill their container completely.

Robert Brown suspended extremely small pollen grains in water and observed it through a microscope. It was found that pollen grains were moving very rapidly throughout the water in a very irregular way. He also observed that warmer the water, faster the pollen grains move on the surface of water. This phenomenon is known as the ‘Brownian Motion’.

The presence of water vapour in air can be demonstrated by the following experiment: We take a steel tumbler and put some well crushed ice in it. Allow the steel tumbler to stand undisturbed for about 5 minutes with the ice in it. We would observe that a large number of tiny drops of water appear on the outer surface of the steel tumbler. This happens because the air around the steel tumbler contains water vapour in it. When these water vapour come in contact with the cold, outside surface of steel tumbler, they condense to form tiny drops of liquid.

The earthen pot (or matka) has a large number of extremely small pores on its walls. Some of the water kept in the earthen pot continuously keeps seeping through these pores to the outside of the pot. This water evaporates continuously by taking the latent heat of vaporization from the earthen pot and the remaining water. In this way, the earthen pot and remaining water loses heat and gets cooled.

Air is a gas whose particles are very far apart and there are very weak forces of attraction between them. Extremely weak forces between particles of air can be overcome easily due to which we can move our hand in air. On the other hand, the particles of a solid plank of wood are very closely packed and there are very strong forces of attraction between the particles of wood. Hence, it needs a huge outside force to overcome the strong inter particle attractions which only a karate expert can apply.

Osmosis is considered to be a special kind of diffusion because it takes place only through a semi-permeable membrane, which allows only solvent particles to pass through it.

1. Osmosis: a, c, d and e.
   
2. Diffusion: b, f and g.
   

This is due to the fact that for melting, each kilogram of ice takes its latent heat of 3.34 × 105 joules from the substance and hence cools the substance more effectively. On the other hand, water at 0° cannot take any such latent heat from the substance.

Bromine diffuses slowly in air because the motion of bromine particles is obstructed due to the collision with the moving air particles. But bromine diffuses very rapidly in vacuum because in vacuum there is no gas, and the motion of the particles of bromine gas is not obstructed by any other particle. Therefore, the diffusion rate in vacuum is high.