CBSE Class 7 Science Advanced Chemistry Water Very Short Answer Questions Set 02

Very Short Answer Questions

Question 1. Distinguish between hard water and soft water.
Answer:
Soft water forms lather freely with soap.
Hard water does not form lather with soap and instead gives a precipitate (scum). It contains bicarbonates, chlorides or sulphates of calcium and magnesium.
In simple words: Soft water makes lather; hard water makes scum.

Teacher's Note:
1. Soap is the test for hardness.
2. Calcium and magnesium salts cause hardness.

Question 2. What is the advantage of distillation by using continuous water still over distillation by using Liebig's condenser?
Answer:
In the continuous water still the water is first warmed by the condensing steam, so less fuel is needed to boil it.
In simple words: It saves fuel.

Teacher's Note:
1. The heat of steam is re-used.
2. That is why it is called a continuous still.

Question 3. Compare the density of water at 4°C with that below and above 4°C.
Answer:
The density of water is maximum at 4°C (1 g/cc). Below 4°C (down to 0°C) and above 4°C the density is less than 1 g/cc.
In simple words: 4°C is the point of highest density.

Teacher's Note:
1. Both below and above 4°C the density is smaller.
2. This is anomalous expansion.

Question 4. What are the values of latent heat of fusion of ice and latent heat of vaporization for water?
Answer:
Latent heat of fusion of ice = 80 cal/g. Latent heat of vaporization of water = 540 cal/g.
In simple words: 80 cal melts 1 g of ice and 540 cal boils away 1 g of water.

Teacher's Note:
1. Learn them as a pair.
2. The temperature stays constant while this heat is taken in.

Question 5. Why is spring water used for medicinal purpose?
Answer:
Spring water has no suspended impurities and contains dissolved salts and rare minerals, which make it useful as medicine.
In simple words: Rare minerals in it have healing value.

Teacher's Note:
1. Spring water is filtered by the ground.
2. The minerals are the reason.

Question 6. Name the substance added to water stored in reservoirs in regions with scanty rainfall. Give reason.
Answer:
Cetyl alcohol is sprayed on the water. It forms a film on the surface that reduces the rate of evaporation, so that water is saved.
In simple words: A thin film stops water from evaporating quickly.

Teacher's Note:
1. It is used on large reservoirs.
2. It is part of water management.

Question 7. What is permutit? Give its formula.
Answer:
Permutit is hydrated sodium aluminium silicate, used to soften hard water. Its formula is \(\ce{Na2Al2Si2O8.xH2O}\).
In simple words: Permutit is a sodium compound that swaps its sodium for the calcium and magnesium of hard water.

Teacher's Note:
1. It is used in the permutit process.
2. It is regenerated with sodium chloride solution.

Question 8. Why is ordinary water a good conductor of electricity?
Answer:
Ordinary water contains dissolved salts, which give ions in water. These ions carry the electric current. Pure water has no such ions and is a poor conductor.
In simple words: Dissolved salts supply the ions that carry current.

Teacher's Note:
1. Distilled water is a poor conductor.
2. That is why a little acid is added in electrolysis.

Question 9. Why is water called universal solvent?
Answer:
Water can dissolve a very large number of substances, more than any other liquid, so it is called the universal solvent.
In simple words: Most things dissolve in water.

Teacher's Note:
1. Even gases like oxygen dissolve in it.
2. Life depends on this property.

Question 10. How are springs formed?
Answer:
Ground water moves through a network of cracks and fissures. At the place where the aquifer meets the ground surface, the water comes out and flows onto the surface under high pressure. This is a spring.
In simple words: A spring is ground water breaking out onto the surface.

Teacher's Note:
1. Springs are natural outlets of underground water.
2. Spring water is free of suspended impurities.

Question 11. What is desalination?
Answer:
Desalination is the removal of dissolved salts from sea water. It is done by reverse osmosis, in which sea water is pressed against a semipermeable membrane at a pressure above its osmotic pressure and only pure water passes through.
In simple words: It turns salty water into fresh water.

Teacher's Note:
1. Reverse osmosis is the method used.
2. Saline water is unfit for use.

Question 12. Define electrolysis.
Answer:
The decomposition of a substance by passing electricity through it is called electrolysis.
In simple words: Splitting a compound using electric current.

Teacher's Note:
1. Electrolysis of acidulated water gives hydrogen and oxygen.
2. It is a chemical change.

Question 13. What is depletion of water table?
Answer:
When more underground water is taken out than is refilled by rain, the level of the water table goes down. This lowering is called depletion of the water table.
In simple words: Drawing out more ground water than gets refilled lowers the water table.

Teacher's Note:
1. It is common in years of scanty rain.
2. Deforestation and population growth add to it.

Question 14. Distinguish between temporary hardness and permanent hardness.
Answer:
Temporary hardness: caused by bicarbonates of calcium and magnesium; removed by boiling or Clark's method.
Permanent hardness: caused by chlorides and sulphates of calcium and magnesium; cannot be removed by boiling; removed by washing soda, the permutit process or ion exchange.
In simple words: Boiling removes the first kind but not the second.

Teacher's Note:
1. Remember the ions: bicarbonate for temporary, chloride and sulphate for permanent.
2. Washing soda removes both.

Question 15. Why is rain water considered as the purest form of natural water?
Answer:
Rain water comes directly from the atmosphere, formed from condensed water vapour, so it has no dissolved salts from the soil. After the first showers have washed out dust and germs from the air, the later rain is free of impurities.
In simple words: Rain water has not touched the ground yet.

Teacher's Note:
1. Ground water and river water pick up salts from soil and rocks.
2. The first rain is dirtier than the later rain.

Question 16. State whether the following statement is true or false: Underground water is free from suspended impurities.
Answer:
True. Underground water is filtered by several layers of soil as it seeps, so suspended impurities are removed.
In simple words: Soil layers filter the water.

Teacher's Note:
1. Dissolved impurities are still present.
2. This is why well water looks clear.

Question 17. State whether the following statement is true or false: Saline water contains large amounts of dissolved salts.
Answer:
True. Water that holds a large amount of dissolved salt, like sea water, is saline water.
In simple words: Saline means salty.

Teacher's Note:
1. Sea water has about 2.5% common salt.
2. It must be desalinated before use.

Question 18. State whether the following statement is true or false: 1 g of pure water at 4°C occupies a volume of 0.97 cc.
Answer:
False. At 4°C the density of water is 1 g/cc, so 1 g of pure water occupies 1 cc.
In simple words: At 4°C water takes exactly 1 cc per gram.

Teacher's Note:
1. 0.97 is the wrong number to remember.
2. Density 1 g/cc is the maximum for water.

Question 19. State whether the following statement is true or false: Water containing calcium bicarbonate produces scum with soap.
Answer:
True. Calcium bicarbonate makes water hard, and hard water forms scum with soap.
In simple words: Hard water gives scum.

Teacher's Note:
1. The scum is the precipitate formed by soap with calcium.
2. Soft water gives lather.