CBSE Class 7 Science Advanced Chemistry Water Long Answer Questions Set 01

Long Answer Questions

Question 1. Explain the electrolysis of water to determine the volumetric composition of water with a diagram.
Answer:
Electrolysis: the decomposition of a substance by passing electricity through it.
Principle: pure water is a poor conductor, so a little sulphuric acid is added. The acidulated water, on passing electricity, decomposes into hydrogen and oxygen in the volume ratio 2 : 1.
\(\ce{2H2O -> 2H2 + O2}\) (on passing electricity)

Process:
1. Fill two test tubes with acidulated water without any air bubbles and invert them over two iron nails connected to the positive and negative terminals of a battery. The nails are the electrodes: the one connected to the positive terminal is the anode and the one connected to the negative terminal is the cathode.
2. Pass electricity for some time. A gas collects in each test tube, and the solution level falls in proportion to the volume of gas formed.
3. Test the gases with a burning splinter. In test tube A the splinter burns brilliantly, so the gas is oxygen. In test tube B the gas burns with a pale blue flame and puts off the splinter with a pop sound, so it is hydrogen.
4. The volumes of oxygen and hydrogen are in the ratio 1 : 2.
Conclusion: the volumetric composition of water is hydrogen : oxygen = 2 : 1. The same ratio is found when water is formed from its elements: \(\ce{2H2 + O2 -> 2H2O}\).
In simple words: Passing electricity through acidulated water gives twice as much hydrogen as oxygen.

Teacher's Note:
1. Draw the diagram and label the anode and cathode.
2. State the tests for the two gases.

Question 2. Give reasons for the following: (a) Sea water is unfit for usage and consumption (b) River water is considered as fresh water (c) Underground water does not contain suspended impurities (d) Distilled water is used for laboratory processes.
Answer:
(a) Sea water has a high proportion of dissolved salts (about 2.5% common salt) along with other impurities, which makes it salty and unfit for use and drinking.
(b) River water flows over land and picks up impurities and soluble salts, but the proportion of dissolved salts is much lower than in saline water, so it is treated as fresh water and is available for many activities.
(c) As rain water seeps through layers of soil, the soil filters out the suspended impurities.
(d) Most chemical reactions take place in water, and laboratory tests need pure water. Distillation removes the dissolved salts, so distilled water is used.
In simple words: Salt makes sea water unfit; soil filters ground water; distillation removes salts for the laboratory.

Teacher's Note:
1. Give one reason in each part.
2. Use the keywords: dissolved salts, filtration, distillation.

Question 3. Explain water cycle and its role in raising the ground water table.
Answer:

Water vapour is added to the atmosphere by evaporation from water bodies, transpiration from plants, breathing of animals and burning of fuels. The vapour, being lighter than air, rises and condenses in the cold upper air into tiny droplets. The droplets join dust particles to form bigger groups (micelles), which appear as clouds. When the clouds hold too much water, it falls as rain. This natural process of adding water to the air and its condensation into rain is the water cycle.
Role in raising the water table: the rain fills the surface water bodies, and a part of it seeps into the ground by infiltration and recharges the ground water. This raises the water table. Thus the water cycle keeps a natural balance in the ecosystem.
In simple words: Water goes up as vapour, falls as rain and a part of the rain soaks into the ground.

Teacher's Note:
1. Draw the cycle with labels.
2. Mention infiltration to link it with the water table.

Question 4. What are the disadvantages of hard water? Explain the various methods of removal of permanent hardness of water with diagrams.
Answer:
Disadvantages of hard water: (i) it is not suitable for drinking; (ii) it gives very little lather with soap, so it is poor for washing, cleaning and bathing; (iii) scale is formed in boilers, which wastes heat and fuel in industries; (iv) it is not good for dyeing, as the dye does not fix on the fabric.
Removal of permanent hardness:
1. Addition of washing soda: it removes both temporary and permanent hardness. The calcium and magnesium salts react with sodium carbonate and form insoluble carbonates that are filtered off.
\(\ce{CaCl2 + Na2CO3 -> CaCO3 + 2NaCl}\)
\(\ce{MgSO4 + Na2CO3 -> MgCO3 + Na2SO4}\)
2. Permutit process: hard water passes through a cylinder filled with permutit, \(\ce{Na2Al2Si2O8.xH2O}\). The calcium and magnesium ions of the water are replaced by sodium ions, which do not stop lather. Exhausted permutit is regenerated with sodium chloride solution.

3. Ion exchange method: the hard water passes first through the cation exchanger A, where positive ions are replaced by \(\ce{H+}\) (or \(\ce{Na+}\)), and then through the anion exchanger B, where negative ions are replaced by \(\ce{OH-}\). The water that comes out is demineralized water. The resins are regenerated with HCl (or NaCl) solution in A and NaOH solution in B.

In simple words: Hard water wastes soap and fuel; permanent hardness can be removed by washing soda, permutit or ion exchange.

Teacher's Note:
1. Draw both the diagrams.
2. Name the hardness-causing salts.

Question 5. Discuss the various steps involved in making potable water.
Answer:
River water has suspended impurities and germs, so it is purified in two stages: removal of suspended impurities and removal of microorganisms.

1. Removal of suspended impurities
(a) Sedimentation: water is pumped into settling tanks and left undisturbed so that the larger suspended particles settle down.
(b) Addition of chemicals: the semi-clear water goes to a mixing chamber where alum and lime are added and mixed with mechanical blades. Alum hastens the settling of the impurities and traps some bacteria. The water then goes to the sedimentation tank.
(c) Filtration: the clear water passes through a sand and gravel filter, which removes the remaining traces of suspended impurities.
2. Removal of microorganisms: bleaching powder is added. It releases chlorine, which kills the harmful bacteria. The water is stored in tanks.
The water thus obtained has no suspended impurities and no harmful bacteria. It still has dissolved gases ( \(\ce{O2}\), \(\ce{CO2}\) ) and some salts, so it is fit for drinking.
In simple words: Settle, add chemicals, filter and kill germs with chlorine.

Teacher's Note:
1. Draw the plant figure.
2. Write the steps in order.

Teacher's Note:
1. Use a table for neatness.
2. Write the pollutant for each cause.

Question 7. (a) Explain the different causes of depletion of water table. (b) What steps should be taken in day-to-day life to prevent depletion of water table?
Answer:
(a) Causes:
1. Scanty rainfall or drought, so that refilling nearly stops while use goes on.
2. Deforestation, which reduces seepage and rain.
3. Increasing population, which reduces the seepage area and increases demand.
4. Increasing industrialization, which increases the use of water.
5. Increased agricultural activities, which depend on ground water.
(b) Steps to prevent depletion:
1. Use water carefully and avoid wasting it (close taps, repair leaks).
2. Harvest rain water and use it to recharge ground water.
3. Plant trees and avoid deforestation.
4. Adopt drip irrigation and closed pipes in farming.
5. Reduce evaporation, for example by cetyl alcohol on reservoirs and plastic covers on plants.
In simple words: Know why the water table falls, and then save water and put rain water back.

Teacher's Note:
1. Give both the causes and the remedies.
2. Link each remedy to a cause.

Question 8. Mention suitable terms for the following statements: (a) Decrease in density of water on decreasing temperature from 4°C to 0°C which is opposite to the normal trend. (b) The underground layer of water bearing permeable rock present below the water table. (c) The natural process of adding water to atmospheric air and its subsequent condensation to rain water. (d) Type of chemical reaction carried out when electricity is passed through water.
Answer:
(a) Anomalous expansion of water
(b) Aquifer
(c) Water cycle
(d) Decomposition reaction (electrolysis)
In simple words: Four key terms of the chapter.

Teacher's Note:
1. Learn all the key terms.
2. Electrolysis is a decomposition reaction.

Question 9. During summer vacations, Aryan went for a trip to Tirupathi in an old car along with his family. During the journey, the driver stopped the car beside a hotel and brought a can full of water and poured it into the radiator of the car. Aryan's son, Mouryan wanted to know the reason for pouring water into the radiator and asked his father who is a mechanical engineer by profession the reason. Aryan explained the role of water in the radiator of the car engine. Can you predict the explanation given by Aryan?
Answer:
The engine of a car gets very hot while running, and this heat must be carried away, or the engine may be damaged.
Water has a very high specific heat capacity, so it can absorb a large amount of heat from the hot engine without a big rise in its own temperature.
The water in the radiator takes away the engine's heat and is cooled again in the radiator. The old car had lost water by evaporation and so had to be topped up.
In simple words: Water takes the engine's heat without becoming too hot itself.

Teacher's Note:
1. High specific heat is the property used.
2. Water is also cheap and easily available.

Question 10. A group of students went for an excursion to Shimla in the month of November. A science teacher also accompanied them. There the temperature fell to −10°C. They took some ice cubes and subjected them to heating while keeping a thermometer in the container. The students monitored the readings of thermometer till the entire ice melted into water. All of them went to their science teacher and showed him their observations. He asked them to discuss among themselves and give a proper justification for the observations on the readings at various stages.
Answer:
Stage 1: the ice at −10°C absorbs heat, and the thermometer reading rises until it reaches 0°C.
Stage 2: at 0°C the ice starts melting, but the reading stays at 0°C until all the ice has melted. The heat supplied is completely used up as the latent heat of fusion (80 cal/g) to change ice into water, and not to raise the temperature.
Stage 3: after all the ice has melted, the heat raises the temperature of the water, and the reading rises again.
In simple words: During melting, the heat goes into changing the state and the temperature stays at 0°C.

Teacher's Note:
1. The temperature stays constant during a change of state.
2. Name the latent heat of fusion.

Question 11. Master Vishnu and his sister Jishnu went to their uncle's village to spend their summer vacation. One day, Jishnu was pestering her uncle for a soft drink. Vishnu's uncle took both of them to a shop where soft drink bottles were kept in an insulated box containing ice. Vishnu's uncle who is a physical science teacher, with an intention of testing Vishnu, posed a question to him regarding the concept involved in cooling of soft drinks by keeping them in ice. Vishnu explained the above phenomenon with respect to the concept of latent heat of fusion of ice. Can you also explain the concept like Vishnu?
Answer:
The latent heat of fusion of ice is 80 cal/g, so every gram of ice at 0°C needs 80 calories of heat to melt into water at 0°C.
This heat is taken from the soft drink bottles that are kept in the ice. So the drinks lose heat and become cold, while the ice melts at a constant temperature of 0°C.
In simple words: Melting ice takes 80 cal/g from the drinks and cools them.

Teacher's Note:
1. Latent heat of fusion is the key term.
2. Ice at 0°C stays at 0°C while it melts.