Science: Air and Water: Layers of Atmosphere and Sources of Water (For CBSE, ICSE, IAS, NET, NRA 2022)

Get unlimited access to the best preparation resource for CBSE/Class-10 : get questions, notes, tests, video lectures and more- for all subjects of CBSE/Class-10.

Layers of Atmosphere

Layers of Atmosphere


It is the undermost layer of the earth՚s atmosphere. At the base of the troposphere, the air is warmer. Density and air pressure also decrease with altitude.


It is the 2nd layer of the atmosphere. It occurs at an altitude of fifty kilometers.


It lies beneath troposphere and above the stratosphere. Temperature decreases with the altitude.


It extends to six hundred kilometers high. Aurora and satellites occur in this layer. It starts just above the mesosphere.

The Ionosphere

It is an abundant layer composed of ionized atoms, molecules and electrons that expand from about forty-eight kilometers above the surface overlapping into the mesosphere and thermosphere.

The Exosphere

It is the upper limit of the earth՚s atmosphere. It extends from the top of the thermosphere up to 10,000 km.

Sources of Water

The natural sources of water are rain, springs, wells, rivers and seas.

  • Rainwater: Rainwater is considered to be the purest form of natural water (distilled water) free from impurities. Water from sea and rivers get evaporated into water vapour by the heat of sun. During this process of evaporation, impurities are left behind. When the water vapours go high up in the air they condense to form clouds. The water drops come down as rain.
  • Spring water: Springs are formed by percolation of rainwater into soil. Springs supply water to wells and lakes.
  • Well water: The rainwater seeps through the soil and goes down and is stored over rocks or hard earth crust. On digging the well this underground water becomes available to us. This is known as well water.
  • River water: Rivers are formed by melting of snow on the mountain, and also sometimes from the rainwater. River water is also not pure and is not fit for drinking.
  • Sea water: it is the source of common salt and other important chemicals. It is the most impure form of water. All the impurities dissolved in river water are carried into the sea. As such, sea water cannot be used for drinking purpose because of high salinity and impurities.

Potable and Non-Potable Water

Potable water means water which is fit for drinking by humans and other animals. It can be consumed with low risk of immediate or long-term harm. Non-potable water is that which is not safe for drinking. It may carry disease causing microbes, and high levels of dissolved salts and minerals, heavy metals and suspended solids.

Purification of Water

  • By decantation: insoluble impurities can be removed. Decantation is the-process of separation of solid from the liquid by allowing the former to settle down and pouring off the latter. Water is kept in a vessel for some time. The suspended insoluble impurities settle down at the bottom. Clean water can now be carefully poured into another clean vessel without disturbing the settled impurities which are left behind. But this water has to be made fit for drinking through further treatment.
  • By filtration: the insoluble impurities can be removed. It is a more effective method than decantation and can remove even very fine particles of insoluble impurities. A piece of clean and very fine cloth can be used as a cheap and easily available filter. When water is poured through it, the insoluble impurities are stopped by the filter and clean water passes through it.
  • By boiling: bacteria and other germs in the water get killed. When boiled water is allowed to cool, heavy impurities collect at the bottom and dissolved salts form a thin layer on the surface called scum. Now if we filter the water, it becomes safe for drinking.
  • By chlorine treatment: small living organisms and bacteria are killed. If required, treated water may be filtered to remove insoluble impurities.

Developed by: