Page 171 - ICSE Chemistry 8

Observa on: There is no residue le behind in the porcelain dish containing rainwater. Concentric rings
of solid residue are observed in the porcelain dish containing well water.
Conclusion: This proves that rainwater does not contain dissolved impuri es whereas well water contains
dissolved impuri es.

Water Cycle

The con nuous circula on of water between land, water bodies and atmosphere cons tutes the water
cycle.

Water is added to the atmosphere by the following processes.

• Water present on the earth’s surface in oceans, seas, rivers, lakes and other water bodies evaporates
due to the sun’s heat and rises upwards into the atmosphere.
• Transpira on by plants releases water to the atmosphere.

• Respira on by living organisms also releases water to the atmosphere.
C H O 6 + 6O 2 6CO 2 + 6H O + Energy
6 12
2
Glucose Oxygen Carbon dioxide Water
• Burning of various fuels, especially fossil fuels adds water to the atmosphere.

Fuel + O 2 CO 2 + H O + Energy
2
Oxygen Carbon dioxide Water
condensation
High up in the atmosphere, air becomes so cool that the water
precipitation
vapour present in it get condensed on dust par cles to form ny
water droplets. evaporation

Many water drops, under suitable condi ons, come together to
form large-sized water drops. Some water drops become so heavy
that they begin to fall on the earth’s surface as rain, hail or snow.
This ensures the con nuous exchange of water between land and
atmosphere. It also controls weather pa erns, and sustains plant
and animal life on the earth.
Fig. 8.4: Water cycle

Take a Break!

Fill in the blanks.
1. ____________ is the purest form of water.
2. The level of water under the ground is called ____________.

3. The con nuous circula on of water between land, water bodies and atmosphere is called
____________.
4. _____________ is the con nuous circula on of water between land, water bodies and atmosphere.

5. _____________ by plants releases water to the atmosphere.

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