Water (CBSE Notes)

Water Cycle : 

Our Earth is covered by two-thirds of water, but most of the water is not potable and contains salt. Water is used for various activities such as agriculture, industries, cooking, cleaning utensils, bathing, washing clothes, and, most importantly, for drinking.
Ponds, wells, streams, lakes and rivers are the different sources of drinking water.They are supplied water by the oceans and seas.Oceans and seas supply water to other water bodies through the water cycle.
The circulation of water from the oceans and the surface of the earth, to the air as water vapour, and its return to the ocean as rain, hail or snow, is called the water cycle


Water cycle: Different steps of water cycle are elucidated.

Evaporation

A variety of salts, like sodium chloride, calcium, magnesium and potassium, are present in saline water. The evaporation of water takes place in the water cycle, leaving the salts behind in the ocean.
The water present on the surface of the ocean evaporates by the sun’s heat. This process of conversion of water from liquid state to vapour state is called evaporation. The sun warms up the surrounding air as well. Evaporation takes place faster in direct sunlight, than in a shady area.
Evaporation also takes place from wet clothes, fields, ponds, lakes and rivers. Plants take in water to grow as well as to prepare their food.

Transpiration

They retain the water they need and release the excess water into the air as water vapour through the stomata of the leaves and the stem. This process is called transpiration. Thus, water is mainly evapo-transpirated to the air from land, water bodies and plants.

Condensation

The evaporated water is carried away by warm air.As the warm air moves higher from the surface of the Earth, it starts to cool down. It is because the water vapour present starts to condense to form tiny water droplets. These droplets float in the air and form cloud and fog. 

Precipitation

All these droplets collect to form bigger drops of water. Some of them may become too heavy to remain in the sky and fall down as rain. This process is known as precipitationIf the air is too cold, the water drops can become snow or hail and may settle on the top of a mountain. When these snow or hail melts, they can become part of a river or a stream. 

Thus, the water that is evaporated from the oceans or seas is again condensed to form water and fills up the rivers and seas. Rain water also seeps into the ground to form ground water. This circulation of water is called the water cycle.


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