Page 79 - ICSE Chemistry 6

Water vapour moves higher up in the atmosphere, cools down and
condenses to form ny drops on small dust par cles. These ny drops
of water together form clouds.
precipitation
When clouds get denser with accumula on of ny drops and become condensation
too heavy, then they fall as rain, hail, sleet or snow.

Rainwater goes back to oceans, seas, rivers, lakes, ponds and streams. evaporation
from plants
Also, water percolates into the soil and is absorbed by plants. evaporation from surface run-off
oceans, lakes, and
The water evaporates again and in this way the water cycle goes on in streams
nature.

Importance of Water Cycle Fig. 4.5: Water cycle
• Water cycle helps to maintain the level of groundwater by
Transpira on
restoring the lost water. It ensures that there is a con nuous
supply of water on the earth. However, when there is rain defi cit The process of giving off water
in the form of vapour through
and over wastage of water in agriculture or human consump on,
the leaves of plants is called
water table falls. The only way out of this is water harves ng. transpira on.

• It helps in controlling the climate of places all over the world.

Change in Density and Volume of Water with Temperature

Anomalous expansion of water: Water has an unusual property. A substance in its liquid state expands
on hea ng under normal condi ons which leads to an increase in the volume of the substance and its
density decreases. When the substance is cooled, it contracts and the density increases. However, in
case of water, it contracts ll 4°C resul ng in an increase in density and decrease in volume. But on
cooling further beyond 4°C, water starts expanding. This results in the increase of volume and the density
decreases ll the temperature reaches 0°C. Water freezes into ice at 0°C. Ice being lighter than water
fl oats in liquid water. This is called anomalous expansion of water.

The density of water is maximum at 4°C. The aqua c animals and plants are able to survive in colder
regions of the world due to this unique property of water. Even when the water in lakes, rivers and ponds
freezes on the top, the water below the ice layer remains in the liquid state.

Activity 4.4

Aim: To show that ice is lighter than water
Materials required: A glass, some water, few ice cubes
Procedure
• Take a glass. Fill half of the glass with water.
• Add a few ice cubes to it.
Observa on: The ice cubes fl oat on water.
Conclusion: This shows that density of ice is less than that of water. In other words, ice being lighter than
water fl oats on water.

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