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SAI INTERNATION AL SCHOOL
Class-7

Lesson Note
Subject-Physics
Ch. - Water: The Life-giving Liquid

Subtopic- Introduction to Water, its form and Water Cycle

Water- A Renewable Resource

 Life on the Earth cannot exist without water.
 More than 70% of the human body is composed of water.
 The human body requires 2-3 litres of water every day for proper functioning.
 Plants and animals also need water for their metabolic functions.
 Water also serves as a Natural habitat for aquatic plants and animals.
Uses of Water
 Water is essential for agriculture, photosynthesis and seed dispersal.
 Water in dams is used to generate electricity.
 Water sports such as swimming, surfing, water skiing , scuba diving and
boating require water.
 Water is used to manufacture medicines and chemicals.
 Water is also effectively used for healing purposes in the treatment of certain
disorders. Water jets are also used in surgery.
 Domestic purposes such as drinking, cooking, washing, cleaning and
gardening require water.

States of water:
 In nature, water occurs in all the three states of matter, solid (ice or snow),
liquid (water) and gas (vapour).
 This three states of water is inter-convertible.

Solid state of water:
 When liquid water is cooled to 0°C (zero degree celsius) it changes to Ice.
 The process by which a liquid changes to its solid form on cooling is known as
freezing.
 The temperature at which a liquid changes to its solid form is known as
freezing point.
 The freezing point of water is 0°C.

Gaseous state of water:
 When liquid water is heated to 100°C it begins to boil and changes into water
vapour.
 The process by which liquid changes to its gaseous form on heating is known
as vaporization.
 The temperature at which a liquid changes to its gaseous form is known as
boiling point.