1. Water And Water Cycle -
ü The most widely used liquid solvent is water.
ü Water is very essential for living beings. Human body has 73% of water.
ü Water cycle refers to circulation of water through the process of evaporation or condensation as rain or snowfall.
ü Water cycle is like a ring. In nature, the water cycle takes place from sea to land and back to sea again.
2. Water Available For Use -
ü About three-fourth of the Earth’s surface is covered with water. That is why it is also called the water planet.
ü Most of the water (about 97%) is in the seas and oceans as salt water. This water is too salty to be used for drinking and irrigation.
ü Thus, only a tiny fraction (about 3%) of the Earth’s water is available to us as freshwater.
ü Out of this, 2.997% is locked up in the mountains or glaciers or is buried so deep under that it costs too much to extract.
ü So, only about 0.003% of the fresh water is easily available to us in the form of groundwater, river, lake, stream, soil moisture, and water vapor.
3. Sources of Water -
ü Rivers and springs - Most of the water which human beings use for drinking, washing and farming comes from rivers and springs. The river flows down the mountain side and across the land, finally flowing into a sea or an ocean.
ü Groundwater - The ground water is actually rainwater which mainly comes from seepage of water accumulated under the ground. Figure 14.3 shows the accumulation of groundwater.
ü 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.
ü Lakes and ponds - These are small reservoirs of water. These are created by Collection of rainwater in low lying areas. Seepage from the groundwater reserves also adds to the water in lakes and ponds.
ü Rain - The rain is a very important source of water for us. All resources of water are fed mainly by rains.
ü Snow - Some regions of the earth are covered with snow especially during winters. Water formed by melting of snow is another source of water. This snow melts slowly. Sometimes this water flows down in the form of streams and rivers. Streams and rivers are another sources of water.
ü Oceans and seas - Most of the water on the earth is found in the oceans and seas. However, the water found in the oceans and seas is not fit for drinking or agricultural purposes as it contains large amount of salt. However, ocean acts as a habitat for large number of plants and animals.
4. Three States of Water -
ü Water can exist in three physical states viz - solid, liquid and gas.
ü Liquid state: The water that we use in everyday life is a liquid. It is called water.
ü Gaseous state: On heating water evaporates to form its vapour. Water vapour is gaseous state.
ü Water turns into ice on cooling. Ice is the solid form of water.
ü These three states of water are inter - controvertible to each other, that is, we can change it from one state to another.
5. Evaporation -
ü The process of converting water to vapour is known as evaporation.
ü Evaporation takes place from open surfaces of water all the time day and night.
ü 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.
ü During the day time, sunlight falls on the water in oceans, rivers, lakes. The fields, roads, rooftops and other land areas also receive sunlight.
ü 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.
ü Thus, water vapour gets continuously added to air due to evaporation.
6. Transpiration -
ü It is the loss of water from the leaves of plants in the form of vapour.
ü 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.
ü The amount of water vapour that goes into air through the process of transpiration is very huge.
7. Condensation
ü The process of conversion of vapour into liquid form of water is called condensation.
ü The process of condensation is opposite to evaporation.
8. Precipitation -
ü 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.
ü Rain - If the water during precipitation remains liquid till it reaches the surface of the earth, we have rains.
ü 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.
ü Hail or 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.
9. Conservation Of Water -
ü Since only a small percentage of water on our planet is usable, it is very important that we use water carefully. Conservation of water can be done by building dams, avoiding wastage of water at homes, both indoors and outdoors, harvesting rainwater, and preventing pollution of water.
ü Building Dam - A dam is a structure built to hold back water in order to prevent floods, and to provide water for irrigation and storage.
ü Dams are also used in producing electricity. Built on rivers, a dam has high walls and has many openings or gates to both let in and hold back water. During heavy rainfall, when the rivers fill up, water enters the dam. When water is needed later, the gates are opened to let out water.
ü Avoiding Wastage at Homes - Some ways to avoid wastage of water at homes are given below.
ü Rainwater Harvesting - The process of collecting and storing rainwater from roofs or a surface catchment is called rainwater harvesting.
ü Storing rainwater that collects on roofs instead of letting it go down the drain, is a practical solution in case of droughts. This technique is known as rooftop rainwater harvesting. This involves collecting rainwater from rooftops in dugout ponds, vessels, or underground tanks to store water for long periods.
ü Another option is to allow water to go into the ground directly from the roadside drains that collect rainwater. The stored rainwater is treated before use because it may contain bird faeces, chemicals, and other pollutants, which need to be removed before use.
ü Preventing Water Pollution - Another method to conserve water is to safeguard our freshwater bodies from pollution.
ü Garbage and harmful chemicals pollute the water and make it unfit for use. Polluted water is also very bad for aquatic life. Plants and animals in and around polluted water may die or get infected. And when human beings consume contaminated fish, etc., they are also put at risk of diseases.
10. Importance Of Water Conservation -
ü It is very important that water should be used carefully. We should take care that water should not get wasted.
ü It is not necessary that the water used in the garden is fit for drinking. Yet most often we water the gardens with drinking water supplied by the corporation. We should use water for gardening that has already been used in the kitchen for washing vegetables and fruits, etc.
ü Always be careful that the water tank in your house doesn’t overflow when it is being filled.
ü Don’t use a hose pipe to wash your car or scooter. Use a bucket instead.
ü If you leave the tap running while brushing your teeth, about 16 litres of water get used up. Fill a mug with water and use instead.
11. Drought -
ü Sometimes it does not rain for a long time for an entire month, two months, the whole season, two years, etc.
ü The abnormally long period of insufficient or no rainfall at all is called drought. During drought, rivers run dry, the water level in lakes goes down, and even the water in the soil dries up.
ü There is very little or no water available during a drought. Most plants die leading to a lack of food in the region.
ü The lack of food in a region for a long period is called famine. When there are no plants, animals that survive on plants also die.
ü Animals and humans also die due to thirst and dehydration (excessive loss of water from the body).
12. Flood -
ü Just as too little rain is bad, too much rain is bad as well. It leads to water being everywhere, sometimes the entire area remains under water or submerged.
ü A condition when the ground becomes submerged under water, due to heavy rain and overflowing of rivers is called flood.
ü During a flood, plants and crops die either due to suffocation by excess water or due to the soil being washed away, robbing their roots of support.
ü In such waterlogged conditions, many disease-causing germs start multiplying and
ü cause water-borne diseases. Sometimes water-borne diseases affect thousands of people at the same time. A disease affecting thousands of people at the same time is called an epidemic.
ü A flood can thus lead to many conditions—disease, starvation, loss of life, and property. A flood also can have the same effect on the food chain as a drought does.
13. Natural Disasters -
ü Both drought and flood are natural disasters—unfortunate events brought about by nature—that can have a very strong effect on the lives of people.
ü The economy of a country can crumble as a result of the loss of property and life and many more things.
ü But we can and must try to reduce the bad effects of these disasters as much as possible.
ü We must understand that such things can happen, and so we must be prepared to face them as and when they happen.
NCERT EXERCISE QUESTIONS
1. Fill up the blanks in the following -
(a) The process of changing of water into its vapor is called _________________.
(b) The process of changing water vapor into water is called _________________.
(c) No rainfall for a year or more may lead to _________________ in that region.
(d) Excessive rains may cause _________________.
Ans. - (a) evaporation. (b) condensation. (c) drought (d) 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.
Ans. - (a) Condensation (b) Evaporation (c) Condensation (c) Evaporation (c) Evaporation
3. Which of the following statements are “true” ?
(a) Water vapor is present in air only during the monsoon. (False)
(b) Water evaporates into air from oceans, rivers and lakes but not from the soil.(False)
(c) The process of water changing into its vapor, is called evaporation.(True)
(d) The evaporation of water takes place only in sunlight.(False)
(e) Water vapor condenses to form tiny droplets of water in the upper layers of air where it is cooler.(True)
4. Suppose you want to dry your school uniform quickly. Would spreading it near an anghiti or heater help? If yes, how?
Ans. - Spreading uniform near an anghiti or heater will help because it increases the rate of evaporation due to release of 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?
Ans. - Because the surface of the air around the bottle cool 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.
Ans. - Air coming out from mouth cool down on the surface of glass hence glass becomes wet.
7. How are clouds formed?
Ans. - 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 vapor 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?
Ans. - If it does not rain for two or more years, water is lost from the soil due to evaporation and transpiration. This dries the soil and the water in the ponds and lakes are dried up, leading to drought condition.
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