NCERT Solutions for Class 6 Science Chapter 15 - Water

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Answer:

The shortage of water is because of the following reasons:

1. Around 97.4% of the total water is present in oceans and seas. This water can not be directly used because of the presence of high concentration of salt. Moreover, some proportion of fresh water is present in glaciers as ice. 

2. The distribution of water is uneven throughout the Earth. Some places have high amount of water, whereas others have very low quantities of water.

3. Water pollution has profoundly degraded the quality of fresh water.

4. The ground water level has profoundly decreased because of human activities. 
 

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Yes, the digestion of food depends on the presence of water. Moreover, water is also required for the absorption of digested food. 

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Solid state of water is ice, whereas gas and liquid states of water are water vapour and liquid water, respectively. 

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In order to convert water from one physical state to another, the physical quantity that should be changed is temperature. 

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The quantity of water in moist air can be decreased by either boiling the container or by using a silica gel that has the ability to absorb water. 

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The rate of evaporation depends on the temperature of the surroundings. Clothes that are hung for drying in the sun experience warmer temperature and thus dry faster. 

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False, air does not become warmer as we go higher up. In fact the air becomes cooler as we go higher up.

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Clouds are made up of water droplets. The water vapour present in the air rises up and are condensed into water droplets on coming in contact with the colder air. These water droplets combine together to form clouds. 

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Rainwater consists of impurities in the form of dust particles, dissolved gases and acids. Dust particles are naturally introduced in the rainwater. Dissolved gases like carbon dioxide are added to the rain water through air. Acids are primarily added to the rain water because of pollution. 

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Rajasthan is a desert area and is located far from the coastal regions. The monsoon winds that carry moisture from the coastal regions gradually loses its moisture when it travel towards Rajasthan. By the time these winds reach Rajasthan, they have already lost most of their moisture and very little rainfall occurs in Rajasthan. Kerala, on the other hand, is a coastal region and thus receives adequate rainfall. 

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Seepage of water will be lesser in a big city. This is because in big cities, open grounds are rarely available and most of the soil is covered with concrete or roads that block the seepage of water in the soil. In villages, open areas and uncovered soils are available which leads to higher seepage of water in the soil. 

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(b) About three-fourths
About three fourths of the earth's surface is covered with water.

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(b) Water to water vapour
On heating water, it changes its form from liquid water to water vapour.

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(b) Groundwater.
Groundwater is the purest form of natural water.

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(d) All of these
All the life process in human beings, such as digestion, excretion etc., require the presence of water.

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(a) Level of groundwater
The level of groundwater is called as water table.

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(d)  Both b and c.
Rainwater harvesting means using rainwater to add to underground water as well as to store rainwater in tanks for other uses like watering plants.

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(c) High salt content.
Water in seas cannot be used because of its high salt content.

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(b) Surface water
Surface water is the rain water which runs off on the land to form lakes, rivers etc.

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(b) dissolved gases
Though rain water is free from germs, it contains dissolved gases present in air.

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True. The roots of the plants absorb water and other nutrients from the soil and transports the same into other parts of the plants. Hence, fruits and vegetables contain a lot of water.

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About seventy percent of the human body consists of water.

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Water in the atmosphere exists in the gaseous state as water vapour.

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True. As the temperature decreases, water changes to ice. As the temperature increases, water changes to water vapour. Hence, change of temperature can be used to change the state of water.

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The conversion of water into ice on cooling is called freezing of water; and the conversion of ice to water on heating is called melting of ice.

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Hot water evaporates faster than cold water. The rate of evaporation depends upon the temperature. Higher the temperature, faster the water evaporates.

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Water evaporates rapidly to form steam when it starts boiling.

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'Water vapour changing to water on cooling' is called condensation.

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True. As the water vapour in the clouds condense, it falls to the ground as rain.

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False. The ground water is pumped out and used extensively for irrigation, drinking, industrial purposes, etc. Also, ground water may flow out naturally into rivers, streams, springs etc., and join the water cycle again.

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True. The water vapour present in the air gets condensed and fall on the earth as rain. If the air contains pollutants, these are also dissolved in the rain water leading to acid rain.

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Water table is the name given to the level at which groundwater is found.

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True. Ground water passes through the soil and surface of the rocks, which filter out the suspended impurities contained in water. Hence, ground water is free from the impurities.

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Sea water cannot be used for irrigation.

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True. The fertile soil on the top most surface of the dry agricultural land are easily blown away by the wind which causes soil erosion. So, scarcity of water during drought increases soil erosion.

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True. Both floods and drought result in the damaging of crops, which in turn leads to the shortage of food and wide spread death of animals and also humans.

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In coastal areas, flooding is mostly caused by cyclones.

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You can collect rainwater for later use by rainwater harvesting.

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The three states of water are:
(i) Ice (solid)
(ii) Water (liquid)
(iii) Water vapour (gaseous)

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Water occurs in nature in solid state as ice, in liquid state as water and in gaseous state as water vapour.

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Evaporation of water occurs at all temperatures. As the temperature increases the rate of evaporation also increase.

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The air gets cooler as it goes higher in the atmosphere. Because of the cooler air, the water vapour gets condensed when it goes higher in the atmosphere.

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Water vapour condenses to form tiny droplets that form clouds. If there are dust particles in the clouds on which the water droplets can settle and get bigger, then the droplets will eventually become large enough to fall as rain drops.

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(a) Water can be changed to ice by reducing the temperature. This is known as freezing of water. (b) Ice can be changed to water by heating it. This is known as melting of ice.
(c) Water vapour can be changed to water by cooling the water vapour. This is known as condensation.

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The ground water which passes along the surface of the non porous rocks, however, at some places come out on the surface, forming a natural spring.

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Following are the uses of water:

1. Water is extensively used for all domestic purposes like drinking, washing, cooking etc.
2. Water is one of the important mediums of transportation.
3. Water a source of energy to produce electricity.

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The cycle which involves the constant circulation of water from earth to atmosphere, and again back to the earth is called as water cycle.

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Water is important for our body because of the following reasons:

1. Water is a major constituent of all bodily fluids such as blood and plasma.
2. Water is essential for digestion of food, absorption of food and also excretion of wastes from the body.
3. Water is useful to cool the body and regulate temperature. When it is hot, we sweat from our skin.
4. A correct concentration of salts is maintained by water, which makes conduction of nerve impulses possible.

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The utility of water to plants is as follows:

1. Water is required for the germination of seeds, their growth and development.
2. The roots of the plants absorb water from the soil. Along with carbon dioxide and water, plants prepare food in the presence of sunlight by photosynthesis.
3. This food is transported to various parts of the plants by means of water.

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To demonstrate the presence of water vapour in air, take some ice cubes in a glass. Let the glass stand without being disturbed for some time. You will observe droplets of water on the outer surface of the glass.
These water droplets result from the condensation of water vapour from the air on the cold surface of the glass. This shows that water vapour is present in air.

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The water table in large cities has been lowered because of the following reasons:

1. In large cities, trees have been felled to make room for buildings and roads causing less rainfall in large cities.
2. In large cities, rain water is diverted by drains into lakes and reservoirs, rather than into the water table.
3. The large population in big cities uses ground water and other water resources faster than they can be replenished.

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Drought is a situation which occurs due to the shortage of rainfall.This results in drying up of crops, which in turn leads to the shortage of food. Hence, during drought, the prices of grains, fruits and vegetables increase.
Floods are caused due to excess rainfall. Due to the excess water flow, the crops gets damaged again resulting in the shortage of food. Again, this causes an increase in food prices.

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During monsoon season, most parts of our country receives heavy rainfall. The rivers which are perennial, receive a very large amount of water. This causes the rivers to overrun their banks and move towards land, causing floods. In coastal areas, floods are often caused by tidal waves, which occur due to cyclones.

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Following are the ways by which we can conserve water and use it effectively:
(i) Using the water effectively, by avoiding wastage and recycling wherever possible.
(ii) Planting more trees and forestation. Trees absorbs water from the ground. Hence, balance the water cycle.
(iii) Storing rain water in large amounts for future use by constructing dams.
(iv) Treating the sewage and industrial wastes before disposing them to the water source, thereby reducing the water pollution.

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Big dams have large water reservoirs to store a huge amount of water at a time. If the rainfall is heavy and the water exceeds the storage limit of the reservoir, then water is released from the dam. This causes heavy flooding, which destroys wildlife, agriculture and displaces people from their homes. Also, large amount of water stored in dams can cause earthquakes in regions prone to tectonic activity.

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Rain water harvesting is storing the rain water for future use. Rain water harvesting can be done in following two ways:
(i) The rain water that falls on the roof of the buildings are directed towards a deep pit in the ground. In this way the ground absorbs the water and thereby adding to the underground water reserves.
(ii) Instead of allowing the rainwater to flow into the drains, the rain water has to be directed to the underground pits to again boost ground water levels.

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If water was not recycled in nature, then the water resources on earth gets depleted. This would cause an imbalance in nature, causing the extinction of life on earth.

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In fast growing towns like Gurgaon, ground water can be recharged by making use of rain water. The rain water that falls on the roof of the building and flowing along the roadside should be directed and allowed to flow into a deep pit in the ground. This water adds to the underground water.

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Water exists in its three states in nature. Other than water, we can also see wax existing in these three states. There are many more elements such as iron, helium, aluminium etc., which can be found in all the three states but under specific conditions of temperature and pressure.

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Some simple habits to save water are -

• Close the tap while brushing
• Use bucket to have a bath instead of a shower
• Check for any kind of leaks
• Close the tap while shaving
• Do not wash the cars with running water 
• Clean cars, bikes and bicycles using waste water from AC's or water purifiers
• Washing the paths, walkways using waste water from AC's or water purifiers
• Use water used to wash vegetables for watering the plants