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Water Cycle

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Water Cycle - Lesson Summary

Water occupies two-thirds of our earth. 97% of water available is salt water found in seas and oceans. At about 3% is the fresh water available on earth. Most of it is in the form of icecaps. Just at about 0.003% of fresh water is available as ground water, lakes, streams, rivers, water vapour etc.

Uses of water
  • 70% of our body is made up of water. Water is essential for normal functioning of life processes.
  • Water is used for various activities such in agriculture, industries and domestic purposes. 70% of fresh water is used for irrigating agricultural fields. 22% of water is utilised by industries. 8% of water is used for domestic requirements.
  • Domestic activities include cooking, cleaning utensils, bathing, washing clothes and mainly for drinking.
  • Ponds, wells, streams, lakes and rivers are the different sources of drinking water. Drinking water is called as potable water. Oceans and seas supply water to other water bodies through water cycle.
  • A variety of useful salts, like sodium chloride, calcium, magnesium and potassium are present in saline water.

States of water
Water exists in three different states. Water can occur in the solid state.
  • Solid state can be represented by ice, snow or hail. Water can occur in the liquid state.
  • Liquid state of water can be represented by river, rain or sea. Water can occur in the gaseous state.
  • Gaseous state of water can be represented by water vapour.

Three states of water are inter-convertible.
  • Solid form of water can be converted into liquid form by heating.
  • Liquid form of water can be converted into gaseous form by evaporation.
  • Gaseous form can be converted into liquid form by the process of condensation.
  • Liquid form can be converted into solid form by freezing.

Water cycle
The cyclic movement of water from the atmosphere to the Earth and back to the atmosphere through various processes is called as water cycle.
Different steps of water cycle include evaporation, transpiration, condensation, precipitation and surface run-off.

a) Evaporation - The water present on the surface of oceans evaporates by the sun’s heat. This process of conversion of water from liquid state to vapour state is called evaporation. Evaporation also takes place from wet clothes, fields, ponds, lakes and rivers.

b)Transpiration - Plants take in water from the soil to prepare their own food and also for other life processes. They release excess water into air in the form of water vapour by the process of transpiration.

c) 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. This water vapour condenses to form tiny water droplets which float in air to form clouds or fog.

d) Precipitation - All these droplets collect to form bigger drops of water. Bigger water drops come down ads rain by the process of precipitation. If 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.

e) Surface run-off – Some amount of rain water is absorbed by the soil and settles down as ground water. Most of the rain water flows down the hills and mountains to collect into rivers, lakes or streams. Rain also washes away the topmost layer of the soil into water bodies.

This circulation of water through all these different factors is called as water cycle.


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