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

Short Answer Questions

Teacher's Note:
1. Quote 4°C and 1 g/cc.
2. This property keeps aquatic life safe in cold lakes.

Question 2. What is meant by infiltration? Explain the role of infiltration in water management.
Answer:
A part of the rain flows into rivers and lakes, while another part seeps into the soil and goes down to the lower layers. Rain water and some river water fill the empty spaces and cracks deep below the ground. This seeping of water into the soil is called infiltration.
Role in water management: infiltration recharges the water table and fills the aquifers, which supply wells, tube wells and springs. Ways of increasing infiltration, such as rain water harvesting and growing trees, keep the ground water level up.
In simple words: Infiltration is rain water going into the ground, and it refills the underground store of water.

Teacher's Note:
1. Mention recharge of the water table.
2. Link it to rain water harvesting.

Question 3. Give some causes of depletion of water table.
Answer:
1. Scanty rainfall or drought: use goes on, but refilling almost stops.
2. Deforestation: fewer trees mean less seepage, quick run-off and less rain.
3. Increasing population: more buildings reduce the area of seepage, and the demand for water rises.
4. Increasing industrialization: industries use huge amounts of water.
5. Increased agricultural activities: farmers depend on ground water for irrigation.
In simple words: We take out more ground water than rain puts back.

Teacher's Note:
1. Give at least four causes.
2. Add one line of explanation to each.

Question 4. Name some agricultural practices which help in water management.
Answer:
1. Drip irrigation: narrow tubes deliver water at the base of each plant, so little water is wasted.
2. Closed pipes instead of open canals: this reduces the loss of water by evaporation.
3. Growing some plants under plastic covers: the water vapour condenses on the cover and returns to the soil.
4. Using natural manure and natural pesticides, which prevents pollution of the available water.
In simple words: Use less water, lose less by evaporation and keep the water clean.

Teacher's Note:
1. Drip irrigation is the most important point.
2. Mention at least three practices.

Question 5. How are bicarbonates added to water? How can they be removed by boiling? Give equations.
Answer:
Addition: carbon dioxide dissolves in rain water. As this water flows over rocks, calcium carbonate and magnesium carbonate in them react with it and form soluble bicarbonates.
\(\ce{CaCO3 + H2O + CO2 -> Ca(HCO3)2}\)
\(\ce{MgCO3 + H2O + CO2 -> Mg(HCO3)2}\)
Removal by boiling: on boiling, the bicarbonates change to insoluble carbonates, which are removed by filtration.
\(\ce{Ca(HCO3)2 -> CaCO3 + H2O + CO2}\) (on heating)
\(\ce{Mg(HCO3)2 -> MgCO3 + H2O + CO2}\) (on heating)
In simple words: Rain water with carbon dioxide dissolves rocks into bicarbonates, and boiling turns them back into insoluble carbonates.

Teacher's Note:
1. Write the equations with the correct formulas.
2. This kind of hardness is temporary.

Question 6. Explain the removal of temporary hardness of water by Clark's method. Give equations.
Answer:
Slaked lime, \(\ce{Ca(OH)2}\), is added to the hard water, as a solid or a solution. It changes the soluble bicarbonates of calcium and magnesium into insoluble carbonates. The carbonates settle down, and filtering gives soft water.
\(\ce{Ca(HCO3)2 + Ca(OH)2 -> 2CaCO3 + 2H2O}\)
\(\ce{Mg(HCO3)2 + Ca(OH)2 -> CaCO3 + MgCO3 + 2H2O}\)
In simple words: Slaked lime turns the dissolved bicarbonates into solid carbonates that can be filtered off.

Teacher's Note:
1. Slaked lime is calcium hydroxide.
2. Clark's method works only for temporary hardness.

Question 7. Give the characteristics of potable water.
Answer:
1. It is free from suspended impurities.
2. It is free from harmful bacteria and other microorganisms.
3. It contains dissolved gases such as \(\ce{O2}\) and \(\ce{CO2}\).
4. It contains some dissolved salts, especially common salt, which help in the body's activities and give taste to the water.
5. It is therefore very suitable for drinking.
In simple words: Potable water is clear, germ-free and has some useful salts and gases.

Teacher's Note:
1. Remember: clean, safe and tasty.
2. Distilled water lacks salts and gases, so it is not potable.

Question 8. Explain the role of alum in the purification of water.
Answer:
Alum and lime are added to water in the mixing chamber, which is then stirred with mechanical blades.
Alum hastens the precipitation of suspended impurities, so they settle quickly in the sedimentation tank.
During the precipitation some bacteria are also trapped and settle down with the impurities.
In simple words: Alum makes the tiny floating particles stick together and sink.

Teacher's Note:
1. Alum is a coagulant.
2. It cannot replace chlorination.

Question 9. Why is distilled water not suitable for drinking?
Answer:
Distilled water has no mineral salts and no dissolved gases.
If we drink it, nutrients from the body fluids dissolve in it and are sent out of the body, which leads to a deficiency of vital nutrients.
It also tastes bland and flat, because the taste and freshness of water come from salts and gases.
In simple words: It is too pure and takes useful minerals out of the body.

Teacher's Note:
1. Pure does not always mean healthy.
2. Potable water is used for drinking.

Question 10. Describe the various stages of purification of drinking water.
Answer:
The purification has two stages: removal of suspended impurities and removal of microorganisms.
1. Sedimentation: river water is pumped into settling tanks and left undisturbed; large suspended particles settle.
2. Addition of chemicals: the semi-clear water goes to a mixing chamber where alum and lime are added and mixed by mechanical blades. It then goes to a sedimentation tank where the impurities settle.
3. Filtration: the clear water is passed through a sand and gravel filter, which removes the last traces of suspended impurities.
4. Chlorination: bleaching powder is added. It releases chlorine, which kills the harmful bacteria. The water is then stored in storage tanks.

In simple words: Settle, add alum and lime, filter, then kill germs with chlorine.

Teacher's Note:
1. Draw the purification plant diagram.
2. Keep the steps in the right order.

Question 11. Draw a neat labelled diagram for the electrolysis of water.
Answer:
Figure: electrolysis of acidulated water in a voltameter.

Acidulated water is filled in the voltameter. Two inverted test tubes (A and B) are placed over the two electrodes. The electrode joined to the positive terminal of the battery is the anode, and the one joined to the negative terminal is the cathode. Oxygen is collected over the anode and hydrogen over the cathode.
In simple words: Two inverted test tubes over two electrodes collect the gases.

Teacher's Note:
1. Label the anode, cathode, voltameter and acidulated water.
2. Mark the gases collected in the two tubes.

Question 12. Explain the process by which both removal of temporary hardness and permanent hardness is possible.
Answer:
Both kinds of hardness are removed by adding washing soda (sodium carbonate). The bicarbonates, sulphates and chlorides of calcium and magnesium undergo double decomposition with sodium carbonate. The insoluble carbonates formed are precipitated and removed by filtration.
\(\ce{Ca(HCO3)2 + Na2CO3 -> CaCO3 + 2NaHCO3}\)
\(\ce{MgSO4 + Na2CO3 -> MgCO3 + Na2SO4}\)
\(\ce{CaCl2 + Na2CO3 -> CaCO3 + 2NaCl}\)
In simple words: Washing soda turns every hardness-causing salt into an insoluble carbonate.

Teacher's Note:
1. Washing soda is the only chemical in the chapter for both kinds.
2. Give one example for each type of salt.

Question 13. Write a short note on water management.
Answer:
Fresh water is limited, so it has to be managed well. Water management has two main parts:
1. Preventing wastage: supply water through leak-free pipes and avoid wasting it in daily life.
2. Rain water harvesting: collecting rain water and using it to recharge ground water, with harvesting systems fitted in houses.
Other steps: drip irrigation in farming, closed pipes instead of open canals, spraying cetyl alcohol on reservoirs to reduce evaporation and growing some plants under plastic covers.
In simple words: Save water and put rain water back into the ground.

Teacher's Note:
1. Mention both main aspects.
2. Add two examples from farming.

Question 14. Draw a neat line sketch of water cycle.
Answer:

Water vapour is added to the air by transpiration from plants, breathing of animals, evaporation from water bodies and burning of fuels. The vapour condenses to form clouds, and rain falls on the ground and into water bodies, recharging the underground water table.
In simple words: Water goes up as vapour and comes down as rain.

Teacher's Note:
1. Label every arrow.
2. Show both rain paths.

Question 15. What is meant by depletion of water table? Explain in detail.
Answer:
The level of the water table goes down when more underground water is taken out than is refilled by seepage of rain water. This is called depletion of the water table.
Ground water is pumped through hand pumps and tube wells and is refilled mainly during rains. When this balance is lost, the level sinks. It is most common when rainfall is scanty.
Causes: drought or scanty rain, deforestation, increasing population, industrialization and heavy use of ground water for farming.
Effect: in the long run it leads to an acute shortage of water. To prevent it, water should be used wisely and rain water harvested.
In simple words: Taking out more water than nature puts back lowers the water table.

Teacher's Note:
1. Define it first, then list causes.
2. End with preventive measures.

Question 16. How can you harvest water in household?
Answer:
Rain water harvesting in a house is done by collecting the rain that falls on the roof.
1. Pipes carry the roof water to a storage tank for later use, or to a pit or well built to let the water soak into the ground.
2. The water that soaks into the ground recharges the ground water and raises the water table.
3. Rain water harvesting systems can be installed in the house for this purpose. Using the stored water for gardening and cleaning also saves the supply water.
In simple words: Catch the roof water and either store it or let it sink into the ground.

Teacher's Note:
1. Roof tops are the usual catchment area.
2. Mention recharge of ground water.

Question 17. Draw a line sketch of distribution of water on earth's surface.
Answer:

Of the earth's water, 97% is saline and 3% is fresh. Of the fresh water, 70% is surface water (rivers 87%, lakes 2% and swamps 11%) and 30% is underground water.
In simple words: Almost all water is salty; only a small part is fresh.

Teacher's Note:
1. Copy the flow chart with the percentages.
2. Rivers hold most of the surface fresh water.

Question 18. List important uses of water.
Answer:
1. It is needed for photosynthesis, by which plants make food.
2. It is the medium for all the chemical reactions of the body of plants and animals.
3. It carries nutrients and minerals to the cells of the living beings.
4. It helps in the exchange of gases through blood during respiration.
5. It is needed for germination of seeds.
6. It is needed for the growth of plants.
7. It helps to regulate body temperature.
In simple words: Water supports every life process.

Teacher's Note:
1. Give at least five uses.
2. Add domestic and industrial uses for extra marks.

Question 19. Discuss five important physical properties of water.
Answer:
1. Nature: colourless, odourless and tasteless liquid.
2. Freezing and boiling points: it freezes at 0°C and boils at 100°C under normal atmospheric pressure (76 cm of Hg). The boiling point rises with the pressure.
3. Density: maximum (1 g/cc) at 4°C; ice is less dense than water. This is anomalous expansion.
4. Specific heat: 1 cal g⁻¹ °C⁻¹, the highest among all substances.
5. Latent heat: latent heat of fusion of ice is 80 cal/g, and of vaporization of water is 540 cal/g.
Other properties: pure water is a poor conductor of electricity, and water is the universal solvent.
In simple words: Say what water looks like, when it freezes and boils, how dense it is, and how much heat it takes in.

Teacher's Note:
1. Pick any five and describe each in a line or two.
2. Always give the numbers.