NOTES
Water cycle: It is circulation of water through the process of evaporation or condensation as rain or snowfall. Water cycle is like a ring. In nature, the water cycle takes place from sea to land and back to sea again.
Evaporation
- The process of changing water to its vapour form is known as evaporation.
- Evaporation takes place from open surfaces of water all the time—day and night.
- Evaporation of water takes place continuously from oceans, rivers, lakes, wells and soil. Oceans, seas, rivers, lakes, ponds and wells together are often known as water bodies.
- During the day time, sunlight falls on the water in oceans, rivers, lakes. The fields, roads, rooftops and other land areas also receive sunlight. The sunlight also carries heat with it. As a result, water from oceans, rivers, lakes and the soil, and other land areas gets continuously changed into vapour.
- Thus, water vapour gets continuously added to air due to evaporation.
Transpiration
- Transpiration: It is the loss of water from the leaves of plants in the form of vapour.
- Every plant, whether it is in a crop field, a forest, on the roadside or in a kitchen garden, transpires to give off water vapour. The amount of water vapour that goes into air through the process of transpiration is very huge.
Condensation
- Condensation: The process of conversion of vapour into liquid form of water is called condensation.
- The process of condensation is opposite to evaporation.
- Cloud formation: The climate close to the earth’s surface is warm. It gets cooled as one goes up in atmosphere. Water vapour being lighter, rises up in the atmosphere. At the upper layer of atmosphere, where the temperature is lower, the vapour gets condensed into tiny water droplets and forms clouds.
Precipitation
- Precipitation: Clouds carry small droplets of water in them. It may so happen that many droplets of water come together to form larger sized drops of water. Such drops of water may become so heavy that they begin to fall. Falling of water drops is called precipitation.
- Rain: If the water during precipitation remains liquid till it reaches the surface of the earth, we have rains.
- Hail/Snow: Sometimes precipitation may be in the form of hail or snow. Water in a hail or snow is in its frozen or solid form.
- Dew: Many times, especially during winter nights, the air near the surface becomes quite cool. As a result, the water vapour present in it condenses to form water droplets. These water droplets appear as dew.
- Water table: If you take soil from ground, it has air as well as water. As you go down, the amount of water increases and air decreases. A level below surface, where it is only water, is called the water table.
Exercise Questions
1. Fill up the blanks in the following:
(a) The process of changing of water into its vapour is called _________________.
(b) The process of changing water vapour into water is called _________________.
(c) No rainfall for a year or more may lead to _________________ in that region.
(d) Excessive rains may cause _________________.
Solution:
(a) The process of changing of water into its vapour is called evaporation.
(b) The process of changing water vapour into water is called condensation.
(c) No rainfall for a year or more may lead to drought in that region.
(d) Excessive rains may cause floods.
2. State for each of the following whether it is due to evaporation or condensation:
(a) Water drops appear on the outer surface of a glass containing cold water.
(b) Steam rising from wet clothes while they are ironed.
(c) Fog appearing on a cold winter morning.
(d) Blackboard dries up after wiping it.
(e) Steam rising from a hot girdle when water is sprinkled on it.
Solution:
(a) Condensation
(b) Evaporation
(c) Condensation
(d) Evaporation
(e) Evaporation
3. Which of the following statements are “true” ?
(a) Water vapour is present in air only during the monsoon. ( )
(b) Water evaporates into air from oceans, rivers and lakes but not from the soil.( )
(c) The process of water changing into its vapour, is called evaporation.( )
(d) The evaporation of water takes place only in sunlight.( )
(e) Water vapour condenses to form tiny droplets of water in the upper layers of air where it is cooler.( )
Solution:
a) False
b) False
c) True
d) False
e) True
4. Suppose you want to dry your school uniform quickly. Would spreading it near an anghiti or heater help? If yes, how?
Solution:
Spreading uniform near an anghiti or heater will help because it increases the rate of evaporation due to heat.
5. Take out a cooled bottle of water from refrigerator and keep it on a table. After some time you notice a droplets of water around it. Why?
Solution:
This is because the surface of the air around the bottle cools down and air condenses around the bottle.
6. To clean their spectacles, people often breathe out on glasses to make them wet. Explain why the glasses become wet.
Solution:
The air we breathe out contains water vapour which condenses on the surface of the spectacles. So, the glass becomes wet and with the help of a small amount of water molecules, it becomes easier to clean the spectacles.
7. How are clouds formed?
Solution:
The process of condensation plays an important role in bringing water back to the surface of the earth. As we go higher from the surface of the earth, it gets cooler. When the air moves up, it gets cooler and cooler. At sufficient heights, the air becomes so cool that the water vapour present in it condenses to form tiny drops of water called droplets. It is these tiny droplets that remain floating in the air and appear to us like clouds.
8. When does a drought occur?
Solution:
If it does not rain for two or more years, water is lost from the soil due to evaporation and transpiration. Since it is not being brought back by rain, the soil becomes dry. The level of water in ponds and wells of the region goes down and some of them may even dry up. The ground water may also become scarce. This may lead to drought.
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