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SAI INTERNATIONAL SCHOOL
Class-7

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

Subtopic- Sources of Water and Types of water
Sources of Water
The main sources of water on the Earth are rainwater, surface water and
underground water.
Rainwater
 The main source of natural water on the earth is rain.
 This is the purest form of water. But, while falling down to the Earth, rainwater
gets polluted with dust particles and some dissolved gases.
Surface Water
 Surface water is the water that collects on the surface of the Earth .
 This includes water present in oceans, seas, lakes, rivers and wetlands.
 Sea and ocean cover almost 70% of the Earth’s surface.
Salinity
Water in the seas and oceans has dissolved salts, especially common salt and
hence is salty or saline. This condition of sea water is called salinity.
Desalination
Removal of salt from sea water and make it fit for drinking and other purposes. This
process is called Desalination.
Groundwater
 Groundwater or underground water is present below the Earth’s surface.
 Rainwater penetrates through the ground and is stored over the non-porous
layer of rocky bed.
 The study of groundwater is known as hydrogeology.
Infiltration

 The process of seeping of water under the ground is called infiltration.
 This reservoir formed above the impervious layers of rock or clay is termed as
aquifer.
Water table
 The area where water fills the aquifer is called the saturated zone.
 The upper layer of this zone is called the Water table.
 Water table may be deep or shallow and may rise or fall depending on many
factors.
 Heavy rains or melting snow may cause the water table to rise while heavy
pumping of groundwater supplies may cause the water table to fall.