Freezing and melting

The process by which water changes from liquid to solid is known as freezing.

The process by which ice or snow changes into water is known as melting.

Let's see how water changes from one form to another with a simple experiment.

• Take an ice-tray and fill it with water. Put it in the freezer. After five hours, take the tray out and the water would have changed into solid ice. This process is called freezing.

• Put a few ice-cubes in an empty glass. Leave the glass out in the sun. After an hour, the ice would have changed into liquid water. This process is called melting.

Four Stages of the water cycle

Water cycle is the process by which water circulates between the sky and the earth in a continuous cycle.

The four stages in this process are:

1. Evaporation
2. Condensation
3. Precipitation
4. Collection

Evaporation

This is the first stage of the water cycle.

The Sun's rays heat the water on the surface of the earth in rivers, oceans and lakes. This makes the water change into water vapour. This process is known as Evaporation.

Here are some activities that we can do to see how evaporation takes place:

• Take a saucer full of water and keep it in the Sun. After a few hours the water in the saucer disappears or lessens.

• Have you ever noticed that wet clothes dry up quickly when spread out in the Sun? Hang a few wet napkins in your courtyard. After some time, the water in the napkins evaporates due to the heat of the Sun.
  
  If the surface area of water is larger, it will evaporate faster. So water kept in a saucer evaporates faster than the same amount of water kept in a glass.

Condensation

After evaporation, condensation occurs. Water vapour is light and rises up towards the sky. Air is cooler high up in the sky. The cool air forms tiny drops of water on dust particles. Many of these dust particles together form a cloud. The process that causes these changes is called condensation.

Let's do some activities that show how condensation occurs:

• Pour some cold water in a glass. You will notice that its outer layer gets covered with tiny droplets.

• Cover a hot cup of tea with a dry plate. When you remove the plate, you will see tiny water droplets on it. The steam that was evaporating from the hot tea changed into water drops when it touched the cool plate.

Precipitation

When these clouds become full and heavy, portions fall to the ground in the form of water or snow. When snow passes through warmer air, it melts and becomes rain.

Collection

After precipitation comes the stage of collection. The raindrops fall back into the lakes, rivers and oceans or are absorbed by the land. This process by which rainwater gathers on earth is called collection.

Water is again heated by the rays of the Sun, causing it to evaporate, condense and fall on earth. Hence the water cycle is a continuous process.