Class -7, Science, Chapter:- 16 Water : A Precious Resource Notes, NCERT Solutions & Frequently Asked Questions

Science

Class:-7, Chapter:- 16

 Water : A Precious Resource

Notes, NCERT Solutions & Frequently Asked Questions

-- Notes --

⭐ Water:- Water is a tasteless,colourless & odourless liquid, which constitutes the major part of the earth. 

• The chemical formula of water is H₂O.

• 71% of the earth consists of water, hence earth is also called as the blue planet.

• 22 March is celebrated as World Water Day. 

⭐ Origin of water:- The origin of water is found out to be from a type of meteorite.

• The major sources of water for human use are lakes, rivers, ponds etc.

• The chemical composition of water in earth is same as that of some meteorites.

⭐ Freshwater v/s saltwater:- 

• Freshwater is the pure & naturally occurring water on earth’s surface in rivers, lakes & underground as groundwater.

• Saltwater is also called as saline water, it is  consists of some amounts of dissolved salts in it. Salt water is present in oceans, seas.

⭐ Statistics of availability of water:- 

• 71 % of Earth is  consists of water. 

• Out of this 71% water, 97% water is in oceans & seas, remaining 3 % is the freshwater in forms of lakes, rivers.

• Out of this 3 % of freshwater, 30% is groundwater (not easily accessible), 69% are in the form glaciers, icebergs ( not accessible).

• Out of the remaining 1% of water only 0.006% is used by the people for their uses.

⭐ Physical properties of water:- 

• Water (H₂O) is a colourless, odourless & tasteless organic liquid made up of hydrogen & oxygen. 

• Water can change its physical state. Eg.- Liquid to solid state and vice versa, Liquid to gaseous state and vice versa.

⭐ Different forms of Water:- 

• Solid State - Water exists in the solid form as ice and snow on the earth.

• Liquid State - Water exist in the liquid state in rivers lakes, oceans and under the ground.

• Gaseous State - Water exists as water vapour in the atmosphere.

⭐ Water cycle:- The process by which water changes its form and circulates between oceans, atmosphere and land is known as the water cycle.

• It is also called as hydrological or hydrologic cycle. 

• The various processes involved in the water cycle are:-

a) Evaporation/Transpiration:- 

• Evaporation is the process, in which liquid phase is changed to a gaseous phase.

• Transpiration is the process by which water evaporates from the surface of leaves through tiny pores called stomata.

b) Condensation:- The process of changing the physical state from gaseous to liquid phase is called as condensation.

c) Precipitation:-  Precipitation occurs as a result of the condensation of water vapour in the atmosphere causing water droplets to become heavy and fall down in the form of rain, snow, hail etc.

⭐ Ground water:- Groundwater is the water present underground or beneath the Earth’s surface in soil spaces and rock formations. 

• Groundwater is generally obtained by digging Wells, using hand pumps and electric pumps.

• Uses of Groundwater:- Groundwater is fresh water and therefore can be used for various purposes such as:

– consumption or drinking water

– domestic chores

– irrigation of crops

– water services in public buildings and offices

– firefighting

– industrial purposes

• Water table – It is the upper limit of the layer of water present beneath the earth. It can vary from one place to another.

– Groundwater is found below the water table. 

– If water is not sufficiently replenished then water table may go down due to increasing population, industries, agricultural activities, decrease of seepage area for seepage of rainwater & scanty rainfall. 

• Infiltration – The process of seeping of water from the surface of soil into the ground is called infiltration.

• Aquifer – The underground layer of rocks that contain water present between their cracks and empty spaces is called Aquifer.

⭐ Effect of water scarcity on plants:- Plants need water to grow by absorbing nutrients from the soil.

• If they do not get enough water, it affects their health and growth eventually killing the plant.

⭐ Water management:- Water management is the effective planning, distribution and use of water resources.

• Some types of water management are as follows:-

a) Rainwater harvesting or water harvesting:- The process of collecting rainwater, which recharges the groundwater is called as rainwater harvesting or water harvesting.

b) Drip irrigation:- Drip irrigation is the process of watering the crops directly to the roots of the plants using narrow pipes.

c) Bawris:-  A traditional way of rainwater storage and recharge.​​​​​​​

⭐ Reasons for scarcity of water :- 

• Deforestation:- Frequent cutting down of trees for human comfortment. 

• Uneven distribution of rainfall.

• Scanty rainfall.

• Increasing industries.

• Increasing population.

⭐ Steps to conserve water:- We can adopt different ways to minimize the wastage of water:-

• Always keep the taps closed while brushing or washing the face to prevent water from flowing away unnecessarily. One should use it only when needed.

• Fix any water taps that are leaking, immediately.

• Instead of taking a shower use a bucket to take bath.

• Use water left from washing clothes for mopping the floor and washing cars instead of running water.

• Water your plans in the morning or evening times so that the water does not immediately get evaporated.

• Do not waste food as it takes a lot of water to irrigate the fruits and vegetables.

• Do not throw away water unnecessarily. Use water in a judicious amount

-- NCERT Solutions -- 

Exercise Questions

Question 1.- Mark ‘T’ if the statement is true and ‘F’ if it is false. 

a) The freshwater stored in the ground is much more than that present in the rivers and lakes of the world. 

b) Water shortage is a problem faced only by people living in rural areas. 

c) Water from rivers is the only source for irrigation in the fields.

d) Rain is the ultimate source of water. (T/F)

Solution:- 

a) True

b) False

c) False

d) False

Question 2.- Explain how groundwater is recharged.

Solution:- The rainwater and water from other sources such as rivers and ponds seep through the soil and fills the empty spaces and cracks deep below the ground. The process of seeping of water into the ground is called infiltration. The groundwater thus gets recharged by this process.

Question 3.- There are ten tubewells in a lane of fifty houses. What could be the long term impact on the water table?

Solution:- If ten tube-wells in a lane of fifty houses are situated, the groundwater will get used for domestic purposes. This results in depletion of groundwater level and water table would go down.

Question 4.- You have been asked to maintain a garden. How will you minimise the use of water?

Solution:- We can reduce the usage of water for gardening by adopting drip irrigation method. In this method, water will reach the roots of the plant drop by drop, which will reduce the wastage of water.

Question 5.- Explain the factors responsible for the depletion of water table.

Solution:- Factors responsible for the depletion of the water table are as follows:

a) Increase in population:-  Increase in population leads to increased demand for water for domestic and drinking purpose. This reduces the water table.

b) Industrialisation:-  Industries need a lot of water for manufacturing purposes. This reduces the water table.

c) Agriculture:- Agriculture requires a lot of water for growing crops; this results in depletion of water table.

Question 6.- Fill in the blanks with the appropriate answers:-

a) People obtain groundwater through________ and ________.

b) Three forms of water are ________, ________ and ________.

c) The water bearing layer of the earth is ________.

d) The process of water seepage into the ground is called ________.

Solution:- 

a) tube wells and hand pumps

b) ice, water and vapour

c) Hydrosphere

d) infiltration

Question 7.- Which one of the following is not responsible for water shortage?

i) Rapid growth of industries

ii) Increasing population

iii) Heavy rainfall

iv) Mismanagement of water resources

Solution:-  iii) Heavy rainfall

Question 8.- Choose the correct option. The total water

i) in the lakes and rivers of the world remains constant.

ii) under the ground remains constant.

iii) in the seas and oceans of the world remains constant.

iv) of the world remains constant.

Solution:- iv) of the world remains constant.

Question 9.- Make a sketch showing groundwater and water table. Label it.

Solution:- 

-- Frequently asked Questions --

Question 1.- What is the origin of water?

Solution:- A study suggested that the water originated from rocks with which the Earth formed.

Question 2.- What are the different types of ‘Condensation’?

Solution:- The types of condensations are:- 

•  Fog 

• Mist 

• Frost 

•Dew

Question 3.- What are the methods that can be followed for proper water management?

Solution:-

• Rainwater harvesting 

• Groundwater recharge 

• Drip irrigation 

• Greywater systems 

• Sewage water treatment

-- By Durgesh Pandey

(Eklavya Coaching Institute)