CBSE Class 7 Science Advanced Chemistry Water Very Short Answer Questions Set 01

Very Short Answer Questions

Question 1. ________ % of the total water available on the earth is saline water.
Answer:
97 (97%)
In simple words: Almost all of the earth's water is in the salty oceans and seas.

Teacher's Note:
1. Only 3% of the earth's water is fresh water.
2. Remember the pair 97% and 3%.

Question 2. Drinking water mainly contains small amounts of salts of ______, ______ and ______ metals.
Answer:
sodium, magnesium and calcium
In simple words: Drinking water has a little sodium, magnesium and calcium salts dissolved in it.

Teacher's Note:
1. These salts give water its taste and help the body.
2. Too much calcium and magnesium makes water hard.

Question 3. Carbon dioxide on dissolution in water forms _______.
Answer:
carbonic acid (\(\ce{CO2 + H2O -> H2CO3}\))
In simple words: Carbon dioxide mixes with water and makes a weak acid.

Teacher's Note:
1. Rain water contains this acid.
2. It reacts with rocks to form bicarbonates.

Question 4. 1 g of water at 100°C on heating forms 1 g of steam at the same temperature by absorbing _________ cal of energy.
Answer:
540
In simple words: The heat taken in to turn 1 g of boiling water into steam is the latent heat of vaporization.

Teacher's Note:
1. The temperature does not change during boiling.
2. 540 cal/g is the figure to remember.

Question 5. 1 g of pure water at 4°C occupies a volume of ________.
Answer:
1 cm³ (1 cc)
In simple words: Water has a density of 1 g/cc at 4°C, so 1 g takes 1 cc.

Teacher's Note:
1. 4°C is the temperature of maximum density.
2. Volume = mass ÷ density = 1 ÷ 1.

Question 6. Water containing calcium bicarbonate produces _________ with soap.
Answer:
scum
In simple words: Hard water does not form lather but gives a curdy precipitate called scum.

Teacher's Note:
1. Calcium bicarbonate makes water hard.
2. Soft water gives lather.

Question 7. _________ volume(s) of hydrogen and __________ volume(s) of oxygen under similar conditions of temperature and pressure combine to give two volumes of water.
Answer:
Two volumes of hydrogen and one volume of oxygen
In simple words: Two volumes of hydrogen join one volume of oxygen and make two volumes of steam.

Teacher's Note:
1. The equation is \(\ce{2H2 + O2 -> 2H2O}\).
2. Volumes are compared at the same temperature and pressure.

Question 8. _________ hardness is removed by Clark's method.
Answer:
Temporary
In simple words: Clark's method uses slaked lime to remove the bicarbonates that cause temporary hardness.

Teacher's Note:
1. Boiling also removes temporary hardness.
2. Permanent hardness needs washing soda or ion exchange.

Question 9. _________ is added to prevent the evaporation of water.
Answer:
Cetyl alcohol
In simple words: A thin film of cetyl alcohol on a reservoir slows down evaporation.

Teacher's Note:
1. It is used where rainfall is scanty.
2. The film acts like a lid on the water.

Question 10. Watering plants by using narrow tubes is called ________.
Answer:
drip irrigation
In simple words: Thin tubes deliver water drop by drop at the roots, so little water is wasted.

Teacher's Note:
1. It is a good method of water management.
2. It also reduces pollution from run-off.

Teacher's Note:
1. Match the sure pairs first.
2. Lake water is fed by rain and rivers.

Teacher's Note:
1. Distilled water fits two entries.
2. River water is not treated, so it is used for irrigation.

Teacher's Note:
1. Link each process to its purpose.
2. Permutit softens hard water.

Question 14. Why does river water contain a lot of impurities?
Answer:
A river flows a long way over land from high ground, so it picks up suspended impurities and soluble salts from the soil and rocks on the way. Waste from towns and industries also gets added to it.
In simple words: A long journey over land and added waste fill river water with impurities.

Teacher's Note:
1. The impurities are of two kinds: suspended and dissolved.
2. River water is fresh, but it is not pure.

Question 15. What is the volumetric composition of water?
Answer:
Water is made of hydrogen and oxygen in the volume ratio 2 : 1 (two volumes of hydrogen to one volume of oxygen).
In simple words: Water has twice as much hydrogen as oxygen by volume.

Teacher's Note:
1. Electrolysis of water proves this.
2. By mass the ratio is 1 : 8.

Question 16. What is the role of water in the evolution of aquatic life?
Answer:
Life began in the oceans. Minerals in sea water reacted in the presence of sunlight, and in the medium of water the first single-celled organisms such as bacteria and fungi arose. Aquatic life evolved from them.
In simple words: The first living things came out of reactions in sea water.

Teacher's Note:
1. Water was the medium for the first chemical reactions of life.
2. Aquatic life is the oldest form of life.

Question 17. Name the most vital process where water takes part in the reaction in which one of the products is a supporter of life.
Answer:
Photosynthesis. Water and carbon dioxide react in the presence of sunlight and chlorophyll to form glucose and oxygen (the supporter of life): \(\ce{6CO2 + 6H2O -> C6H12O6 + 6O2}\).
In simple words: Plants use water to make food and give out oxygen.

Teacher's Note:
1. Oxygen is the supporter of life here.
2. Write the full equation for full marks.

Question 18. What is an aquifer?
Answer:
An aquifer is the underground layer of water-bearing permeable rock or loose material below the water table, from which ground water can be taken out through a well.
In simple words: An aquifer is an underground layer that holds water.

Teacher's Note:
1. It lies below the water table.
2. Wells and tube wells draw water from it.

Question 19. Underground water does not contain suspended impurities. Why?
Answer:
While rain water seeps down through the layers of soil, the soil filters it and holds back the suspended particles.
In simple words: The soil works like a natural filter.

Teacher's Note:
1. Seepage is a kind of filtration.
2. Dissolved salts, however, still remain in it.

Question 20. What is meant by rain water harvesting?
Answer:
Rain water harvesting is collecting rain water and using it to recharge the ground water, for example by fitting harvesting systems in houses.
In simple words: Catching rain water and sending it into the ground.

Teacher's Note:
1. It raises the level of the water table.
2. It is an important part of water management.