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NCERT Solutions for Class 7 Science Chapter 16: Water A precious resource



Introduction

One of the main reasons for the presence of life on Earth is water. Water is one of the most essential constituents of life.
Water-a renewable natural resource: Natural resources such as water and air which are constantly renewed through Rapid natural cycles are called renewable natural resources.
The continuous evaporation of water into the atmosphere, formation of clouds and precipitation as rain and snow replenishes freshwater on the Earth.
Water in different forms:
Water exists in three different forms: solid, liquid and gas.
  • The polar regions of the Earth and mountains have ice. Air contains water vapour which is the gaseous state of the water. The three forms of water are interchangeable
  • Ice is the solid form of water. It melts at 0 degree celsius and changes into liquid water,, the process called melting.
  • Water, which is a liquid, boils at 100 degree celsius and changes to steam by the process called vaporization.
  • Water vapour changes to liquid water when the temperature decreases or it is cooled, the process known as condensation.
  • Liquid water solidifies into ice when it is further cooled by the process called freezing.

In nature, water takes the forms known with different names are as follows:

Solid
Liquid
Gas
Glaciers Icebergs
Snow
Hail
Frost
Rain
Dew

Fog
Steam
Water vapour

Glacier: It is a large chunk of ice that moves like a river over the surface of land.
Iceberg: It is a large piece of ice that has broken off a Glacier and floats in seas and oceans.
Snow: It is a form of precipitation in which a crystal of ice falls on the ground.
Hail: These are hard lumps of ice falling from the sky during precipitation.
Frost: it is formed when water vapour solidifies and gets deposited on plants, trees, homes, vehicles etc.

Water cycle:

The continuous circulation of water from the Earth's surface to the atmosphere by evaporation and back to the earth by condensation is known as the water cycle.
  • Water from water bodies and moist soil evaporates and rises up in the air in the form of water vapour (gaseous state). Plants also give out water from their leaves in high amount as a result of transpiration.
  • When this vapour rises up, it condenses to form clouds, in which the vapour cools down to form water droplets.
  • When the clouds get too saturated with water, they pour down as rain (liquid state), by the process called precipitation.
  • In colder regions, water drops freeze and fall as hail or snow (solid state).
  • The rainwater that falls into seas, rivers, lakes etc. increases their water level. The water from these water bodies evaporates back into the atmosphere, completing the water cycle.


Uses of water:

Body functions
Plants need water to prepare their own food and for germination of seeds. Animals and human beings need water for digestion and absorption of nutrients.
Domestic use
We need water for drinking, cleaning, cooking, bathing, washing, etc.

Agricultural use
Crops like paddy, wheat, sugarcane, etc. need a large amount of water to grow.
Industrial use
Water from river is used to produce electricity.

Use in transport
Huge ships transport goods containers and people all around the world through rivers, seas and oceans.
As a habitat
Different freshwater and marine water bodies are the habitat for different kinds of plants, animals, fish and microorganisms.



Sources of water:

Rain is one of the main sources of freshwater. Some of the rainwater that falls on the land surface may flow into rivers, streams or oceans. This is called runoff water. The runoff water either flows into rivers or gets stored in lakes to form a freshwater source.

Surface water: After rainfall, water gets collected in the low- lying areas to form ponds and lakes. In hilly regions, the runoff water falls from higher areas to form rivers that flows into valleys. The rivers further flow two seas or oceans. All these water sources form the surface water of the earth.
Underground water: During the summer season or in case of failure of rainfall, people depends on the water from underground sources for domestic and agricultural purposes.
Rain water gets collected in the soil by seeping into the gravel and rocks at the bottom. The excess amount of water moves deeper into the ground and fills the spaces in the rocks. This is known as groundwater. The level of groundwater is known as the water table.
Aquifer: The rainwater that has collected in the empty space in the porous layer of rock is known as an aquifer. The water remain there because below it is a hard layer of rock which prevents water from moving further downwards.


Depletion of water table (Scarcity of water):

In many areas, groundwater is nearly exhausted. This is because it is being used at a faster rate than it is being renewed by the natural process. The increase in use is due to increase in human population industries and agriculture.
  1. Increase in population: An increase in the population creates a greater demand for the construction of houses, shops, offices, Industries, roads,etc. leading to overuse of the water resources and thus water scarcity.
  2. Increase in industries: Industries dealing with thermal Power, textiles, paper, chemicals and metals use huge quantities of water. The use of water as a coolant in industries and for washing causes scarcity and leads to pollution of water sources.
  3. Agricultural activities: Nowadays, farmers use large quantities of groundwater by sinking deep tube Wells to cultivate crops. Uncontrolled increase in the number of tube wells in a region will deplete groundwater in the near future leading to acute water shortage.
  4. Water pollution: Contamination by harmful chemicals and leaking septic tanks cause groundwater pollution. Polluted water is unfit for human consumption.

Consequences of Water Scarcity:

On humans: Unavailability of clean drinking water, which leads to health problems and unhygienic conditions.
On plants: Plants need water to carry out photosynthesis. They wither and die when there is a lack of water.
On land: The soil dries up. The land eventually becomes barren when there is no availability of water methods of conservation of water.

Methods of Conservation of Water:

Economic use and conservation of water are the two main methods to save water.
(1)Rainwater harvesting: A lot of rain water goes into storm drains during the rainy season.This water can be conserved for use or to replenish the groundwater levels by the method of rainwater harvesting. Rainwater harvesting can be done by collecting rainwater from rooftops and storing it in tanks. The water can be used for various household purposes. Rainwater harvesting can raise the underground water table of the region.

(2) check dams: Rainwater percolates into the soil only if it does not flow away quickly. Water can be prevented from flowing away by constructing check dams in the path of its flow.
(3) Drip irrigation: The drip irrigation system supplies the required amount of water only around the roots. Since water is supplied drop by drop, water loss through evaporation and runoff is prevented thus saving water.
(4)A huge amount of potable water is lost during the distribution of water because of leakage in the main water pipelines. Fixing these leakage can save a lot of water from going into the drains.
(5) Care should be taken by not to pollute the lakes and ponds by using them for washing clothes, bathing animals and other human activities.
(6)Another simple and effective method to reduce the usage of water is by recycling the water from industries and using it for horticulture purposes.

Distribution of water in India:

  • The movement of wind influences rainfall in India. Rainfall varies from place to place and the distribution of water is uneven.
  • Some regions such as those of Rajasthan, Northern Karnataka and Gujarat do not have sufficient rainfall and are prone to drought, whereas some regions such as Andhra Pradesh, Orissa, West Bengal receive excess rain causing floods.
  • Dams are built to maintain and distribute water supply. Artificial walls are built across the river that control the flow of water in them. It holds a reservoir whose water content is maintained by the opening and closing of the gates.
  • The Bhakra-Nangal dam is built on the river Satluj and provides water to the states of Punjab and Haryana.
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Also Read



Reference Books for class 7 Science

Given below are the links of some of the reference books for class 7 math.

  1. Science for Class 7 by Lakhmir Singh
  2. Science Foundation Course For JEE/NEET/NSO/Olympiad - Class 7
  3. CBSE All In One Science Class 7 by Arihant Experts (Author)
  4. IIT Foundation Physics, Chemistry & Maths for Class 7

You can use above books for extra knowledge and practicing different questions.



Class 7 Maths Class 7 Science





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