Water – CBSE Notes for Class 6 Science
CBSE NotesCBSE Notes Class 6 ScienceNCERT Solutions Science
Water Available For Use:
About three-fourth of the Earth’s surface is covered with water. That is why it is also called the water planet. But do you know how much water is readily available for use? 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. Thus, only a tiny fraction (about 3%) of the Earth’s water is available to us as freshwater. 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.
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 vapour. See Figure 15.1.
Uses Of Water :
Besides being essential for life, water is used for many other purposes. In India, about 70% of the total water available is used for agriculture, 20-22% by industries, and only 8% is used for personal or domestic needs. Figure 15.2 shows a pie chart that gives the percentage use of water.
Let us learn more about the various uses of water.
Agricultural needs Our country depends a lot on agriculture. Farmers rely on water to sustain their agricultural crops, e.g., wheat, paddy, etc. Many a times, rainfall is not sufficient to water these crops, and farmers have to use artificial watering systems, referred to as irrigation.
Industrial needs Factories use a large amount of water every day—as raw material, for cleaning, heating, cooling, generating electricity (e.g., water turbines), etc. The amount of required depends on the kind and size of the factory, and water.
Personal/domestic needs We need water to drink. Water that is suitable for drinking is called potable water. We also need water to bathe, wash clothes and dishes, clean our house, and to water plants.
Apart from these uses, water is also used for transportation and recreation (Fig. 15.6). It also regulates the climate of a place and provides homes to many animals.
Different States Of Water :
In nature, water exists in three states. It could be in the form of liquid (e.g., rain, river, sea), solid (e.g., ice, snow, hail), or gas (e.g., water vapour) (Fig. 15.7).
You can heat water over a stove to convert it into vapour. What happens if you leave water in an uncovered vessel on a summer afternoon outside your house? After a few hours, you will find that the level of water in the vessel has decreased. This is because a lot of it would have escaped into the atmosphere in the form of water vapour. The process by which a liquid is converted to its vapour is called evaporation.
What about the reverse process? The process by which the vapour of a substance is converted to its liquidform is called condensation. Water vapour is also added into the air by the leaves of plants, through the process of transpiration.
Evaporation and condensation of water take place on a very large scale on the surface of the Earth and its atmosphere. These processes play a key role in cloud formation and rain.
Cloud Formation :
When the temperature of air increases, it expands (i.e., its particles move away from one another). This makes the air lighter and it rises in the atmosphere, taking water vapour with it. As the air rises, it begins to cool. The water vapour condenses on dust particles present in the atmosphere to form millions of tiny droplets. Tiny ice crystals will be formed instead if it is very cold. This cluster of tiny water droplets floating in air is what we call a cloud.
Water Cycle :
Water droplets in the clouds keep bumping against one another, and sometimes stick to form bigger drops. When these drops become too heavy to float in the air, they drop down back to the Earth as rain. The water that comes down as rain, in time, evaporates and goes up to form clouds again. This leads to form a cycle, known as the water cycle.
Water cycle is the cyclic movement of water from the atmosphere to the Earth and back to the atmosphere through various processes.
Figure 15.9 shows how the water cycle works.
Evaporation Sun’s heat changes the water in oceans, rivers, ponds, and other water bodies into water vapour. Transpiration Plants give off water vapour through their leaves.
Condensation Water vapour rises up and condenses on dust particles to form clouds.
Precipitation Water stored in clouds reaches Earth in the form of rain, snow, etc.
Collection Some rainwater seeps into the ground, forming ground water. Rainwater also flows into streams and rivers, and then finds its way into seas and oceans.
Drought :
Sometimes it does not rain for a long time—for an entire month, two months, the whole season, two years, etc. The abnormally long period of insufficient or no rainfall at all is called drought. During drought, rivers run dry, water level in lakes goes down, and even the water in the soil dries up (Fig. 15.10).
There is very little or no water available during a drought. Most plants die leading to lack of food in the region. 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. Animals and humans also die due to thirst and dehydration (excessive loss of water from the body).
Flood :
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. A condition when ground becomes submerged under water, due to heavy rain and overflowing of rivers is called flood.
During a flood, plants and crops die either due to suffocation by excess water or due to soil being washed away, robbing their roots of support.
In such waterlogged conditions, many disease-causing germs start multiplying and
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.
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.
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. The economy of a country can crumble as a result of loss of property and life and many more things. But we can and must try to reduce the bad effects of these disasters as much as possible. We must understand that such things can happen, and so we must be prepared to face them as and when they happen.
Conservation Of Water :
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.
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.
Building a dam is a solution to both drought and flood (Fig. 15.12).
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.
Avoiding Wastage at Homes:
Some ways to avoid wastage of water at homes are given below.
Rainwater Harvesting:
The process of collecting and storing rainwater from roofs or a surface catchment is called rainwater harvesting (fig. 15.14). 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. 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.
Preventing Water Pollution:
Another method to conserve water is to safeguard our fresh water bodies from pollution. Garbage and harmful chemicals pollute the water and make it unfit for use. Polluted water (Fig. 15.15) 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 the contaminated fish, etc., they are also put at risk of diseases.
Key Words :
Salt water: Water of the seas and oceans that has a high salt content, making it unfit for drinking and irrigation, is called salt water.
Fresh water: Water found in rivers, lakes, and ponds used for domestic and commercial purposes is called fresh water.
Irrigation: Watering crops by artificial means is called irrigation.
Potable water: Water fit for human consumption is called potable water.
Transpiration: The release of water vapour into the atmosphere through the leaves of plants is called transpiration.
Water cycle: The cyclic movement of water from the atmosphere to the Earth and back to the atmosphere through various processes is called water cycle.
Drought: Abnormally long period of insufficient or no rainfall is called drought.
Famine: Lack of food in a region for a long period is called famine.
Flood: A condition when ground becomes submerged under water, due to heavy rain and overflowing of rivers is called flood.
Epidemic: A disease affecting thousands of people at the same time is called epidemic.
Dam: A structure built on a river to store and hold back water is called a dam.
Rainwater harvesting: The process of collecting and storing rainwater from roofs or a surface catchment is called rainwater harvesting.
Summary :
Only a tiny fraction of the Earth’s water is available as fresh water. All living things have a lot of water in their body. Almost 70% of our body weight contains to water.
• We need water for many purposes—drinking, personal needs, agricultural needs, industrial needs, for transportation and recreation, and regulating the climate. It is home to various plants and animals.
• Water exists in solid, iiquid, and gaseous states.
• Evaporation and condensation play an important role in cloud formation.
• Water cycle is the cyclic movement of water from the atmosphere to the Earth and back to the atmosphere through various processes.
• Abnormally long period of insufficient or no rainfall is known as drought.
• When there is too much rainfall in an area, rivers overflow and water covers all the area around. This is called a flood. A flood can cause great destruction.
• Rainwater harvesting is one of the ways to conserve water.
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