Friday, January 27, 2012

VII Science Chemistry : Water : A Precious Natural Resource

The year 2003 was observed as the INTERNATIONAL YEAR OF FRESHWATER to make people aware of this dwindling natural resource. 22nd MARCH is celebrated as the WORLD WATER DAY.
NCERT CBSE Class VII Water ‘A Precious Natural Resource’
Q. Fill in the blanks with the appropriate answers:
(a) People obtain ---------through tube wells and hand pumps.[groundwater]
(b) Three forms of water are solid, liquid and ---------[vapour.]
(c) The water bearing layer of the earth is ----------[ aquifer]
(d) The process of water seepage into the ground is called ---------[ infiltration.]
(e) The process of changing of water into its vapour is called---------[ evaporation.]
(f) The process of changing water vapour into water is called ---------[condensation.]
(g) No rainfall for a year or more may lead to ---------- in that region.[drought]
 (h) Excessive rains may cause ----------- [flood.]
Q. So much of water available on our planet Earth, but why do we still feel a shortage of water?
Ans: 75% of our Earth is made of water but only 3% of it is fresh water. Hence only a fraction is fit for human consumption and we still feel a shortage of water.
Most of the water that exists on the earth is in the seas and oceans which is highly salty.
Q. What are main sources of water?
Ans:    a. Rainwater                          b. Glaciers, ice and snow
           c. River water                      d. Sea and Ocean water
          e. Lake and Pond water
Q. What are three states of water?
Ans: As Solid – In the form of ice crystals, snow, ice pellets, hail, and frost
As Liquid: In the form of rain and dew
As Gas : In the form of Water vapour
Q. What is wter cycle?
Ans: The continuous circulation of water in nature is called water cycle. It is also called the hydrological cycle
Q: What is water table?
Ans: A level where all the space between particles of soil and gaps between rocks are filled with water. The upper limit of this layer is called the water table.
Q. What do you mean by aquifer?
Ans: Some places groundwater is stored between layers of hard rock. This is called aquifer. Water in the aquifers can usually be pumped out with the help of tube wells or hand pumps .
Q. What are causes of depletion of water table?
Ans: a. Scanty rainfall and hot winds are natural forces that may deplete the water table
b. Deforestation, increased population, rapid urbanization, overgrazing by cattle, excess tapping of ground water are human causes that deplet water.
c. Commercialization of water resources.
Q. What is desalination. Explain.
Ans: Desalination is an artificial process by which saline water ( sea water) is converted to fresh water.
The most common desalination processes are :
1. Distillation :The process in which both evaporation and condensation go side by side is called distillation. Water obtained through distillation is called distilled water. This water is normally pure enough for use in school science and medical laboratories.
2. Reverse osmosis: The process of forcing water under pressure through a semi permeable membrane whose tiny pores allow water to pass but exclude most salts and minerals.
Q. How can we conserve water?
Ans: Water conservation is process of preventing wastage of water, using water carefully and recharging ground water. Water conservation can be done by :-
i) Repairing leaking pipes and taps.
ii) Not wasting water during brushing teeth, shaving, bathing, washing clothes and during other activities.
iii) Rainwater harvesting.
iv) By drip irrigation of plants.
Q. What do you understand by anomalous expansion of water?
Ans: When water at room is cooled, it contract until reaches 40c and then starts expanding. This strange behavior of water is called anomalous expansion of water.
Q. Why ice floats on water?
Ans: Ice is lighter than water so floats on water.
Q. Why water piper burst cooler part of world in winter?
Ans: Water keeps expanding from 4C to 0 C and occupies more space in form of ice and pipe burst out.
Q. What is specific heat capacity of substance?
Ans: The amount of heat required to raise the temperature of substance is known as specific heat capacity of substance.
Q. Why water used as an excellent cooling agent in car engine?
Ans: Water has high specific heat capacity of substance and takes longer time to heat up.
Q. Why water is called universal solvent?
Ans: A large number of substances dissolve in water so it is called universal solvent. A liquid that dissolve another substance (solute) to form solution is called solvent.
Q. What is drip irrigation?
Ans: It is a method of watering plants using narrow pipes through which water drips into the base of the plants

Further reading links 

NCERT CBSE Class VII Science Chemistry : Water : A Precious Natural Resource
Water : Precious Natural Resource 
Water : Precious Natural Resource 
Waste water story     
Electric Current and Circuit      
Light Mirror and Lens              
Pollution Of Air And Water    

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