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

Short Answer Questions

Question 1. Describe various sources of water.
Answer:
Water is available as surface water and underground water.
1. Oceans and seas: the largest source (about 97%), but the water is saline.
2. Rivers and lakes: fed by rain and melting snow; fresh water with impurities and soluble salts.
3. Rain water: the purest natural water; the later showers are free from impurities.
4. Underground water: rain water that seeps into the ground. It includes well water, drawn with hand pumps and tube wells, and spring water, which flows out of the ground under pressure. It has no suspended impurities.
In simple words: Water comes from oceans, rivers, lakes, rain and from under the ground.

Teacher's Note:
1. Divide the answer into surface and underground water.
2. Add one fact about each source.

Question 2. A group of students accompanied by their science teacher went trekking. After reaching the top of the hill, they felt hungry. The teacher told them that they would have to bear the hunger as cooking of food is slow and takes a longer time at the top of the hill. The students wanted to know the reason for the above fact. The teacher explained the reason for the above fact. Can you predict the explanation given by the teacher?
Answer:
As we go to a higher altitude the atmospheric pressure decreases.
The boiling point of water falls when the pressure falls, so on a hill top water boils at a temperature lower than 100°C.
Food cooks slowly in this cooler boiling water, so cooking takes a longer time.
In simple words: Low pressure on a hill makes water boil at a lower temperature, so food takes longer to cook.

Teacher's Note:
1. Boiling point goes up and down with pressure.
2. A pressure cooker does the reverse.

Question 3. In a science exhibition, a student called Rishi performed the following experiment. She took two containers of the same dimensions and filled one of them with wax and the other one with ice. She placed both of them in a boiling water bath. What difference could she observe during the melting of these two substances? Also predict the reason for the observed difference.
Answer:
The molten wax overflows from its container, while the water formed from the ice does not overflow (its level falls).
Reason: solid wax is denser than molten wax, so wax expands on melting and the volume increases. Ice is less dense than water, so ice shrinks on melting and the water occupies less volume.
In simple words: Wax grows in volume on melting, but ice shrinks.

Teacher's Note:
1. Ice is the exception to the rule.
2. This is related to the anomalous expansion of water.

Question 4. Why is sea water not suitable for drinking? How is it made potable?
Answer:
Sea water contains a large amount of dissolved salts (about 2.5% common salt), so it is unfit for drinking.
It is made suitable by desalination using reverse osmosis: pressure higher than the osmotic pressure is applied on sea water kept against a semipermeable membrane, so pure water passes through and the salts remain behind.
Distilling sea water and adding a little common salt to the distilled water also makes it potable.
In simple words: Remove the salt from sea water to make it drinkable.

Teacher's Note:
1. Name reverse osmosis.
2. Distillation is another way.

Question 5. Hot water bags are generally used for reducing body pain. Can they also be used to keep us warm in winter? Explain.
Answer:
Yes. Water has a very high specific heat capacity, so it can give out a lot of heat while cooling only slightly.
The heat given out by the hot water bag raises the temperature of our body and the objects in contact with it, so it keeps us warm in winter.
In simple words: Hot water holds a lot of heat and gives it out slowly.

Teacher's Note:
1. High specific heat is the key idea.
2. The same idea explains why water is used as a coolant.

Question 6. Though ice is the solid form of water, it floats on the surface of water. Justify.
Answer:
The density of ice (0.91 g/cc) is less than the density of water. Anything less dense than the liquid floats on it, so ice floats on water.
In simple words: Ice is lighter than water for the same volume.

Teacher's Note:
1. Water expands when it freezes.
2. About 9/10 of the floating ice stays under water.

Question 7. Calculate the amount of heat energy required to increase the temperature of 10 g of water through 12°C.
Answer:
Specific heat of water = 1 cal g⁻¹ °C⁻¹.
Heat required = mass × specific heat × rise in temperature = 10 × 1 × 12 = 120 cal.
In simple words: 10 g needs 10 cal for each degree, and there are 12 degrees.

Teacher's Note:
1. Use Q = m × s × t.
2. Give the unit calorie.

Question 8. Calculate the ratio of the amount of heat energy lost during the conversion of 20 g of water at 0°C to ice at 0°C to heat energy gained during the conversion of same amount of water at 100°C to steam at 100°C.
Answer:
Heat lost when 20 g of water at 0°C changes to ice = 20 × 80 = 1600 cal.
Heat gained when 20 g of water at 100°C changes to steam = 20 × 540 = 10800 cal.
Ratio = 1600 : 10800 = 4 : 27 (about 1 : 6.75).
In simple words: Multiply the mass by 80 for freezing and by 540 for boiling, then divide.

Teacher's Note:
1. Use the latent heat values 80 and 540.
2. Simplify the ratio.

Question 9. Two open containers A and B are filled with water and in container A, cetyl alcohol is added. Both the containers are placed at 40°C. Predict the level of water in the two containers after some time and give reasons.
Answer:
The level of water in container A will be higher than in container B. Cetyl alcohol spreads as a thin film over the water in A and reduces the rate of evaporation, so A loses less water, while the water in B evaporates faster.
In simple words: The film slows down the loss of water.

Teacher's Note:
1. Cetyl alcohol is used on reservoirs for the same reason.
2. Higher temperature increases evaporation.

Question 10. The electronic configurations of two metals X and Y are 2, 8, 2 and 2, 8, 8, 2, respectively. What is the effect of the presence of hydrogen carbonate and sulphates of these metals in water?
Answer:
X (2, 8, 2) is magnesium and Y (2, 8, 8, 2) is calcium.
Hydrogen carbonates (bicarbonates) of calcium and magnesium cause temporary hardness, while their sulphates cause permanent hardness.
In simple words: The first kind of salt gives temporary hardness and the second kind gives permanent hardness.

Teacher's Note:
1. Find the elements from the configurations first.
2. 12 electrons is Mg and 20 electrons is Ca.

Question 11. An element with electronic configuration 2, 8, 1 forms a compound with carbonate. How does this compound remove both temporary and permanent hardness of water?
Answer:
The element 2, 8, 1 is sodium, and its carbonate is sodium carbonate (washing soda).
It reacts with the bicarbonates, chlorides and sulphates of calcium and magnesium and forms insoluble carbonates, which settle and are filtered.
\(\ce{Ca(HCO3)2 + Na2CO3 -> CaCO3 + 2NaHCO3}\)
\(\ce{CaSO4 + Na2CO3 -> CaCO3 + Na2SO4}\)
So washing soda removes both temporary and permanent hardness.
In simple words: Washing soda turns calcium and magnesium salts into solid carbonates.

Teacher's Note:
1. Sodium has 11 electrons.
2. Give one equation for each kind of hardness.

Question 12. In permutit process, what is the role of the addition of the concentrated solution of the solid compound obtained due to the reaction between NaOH with HCl? Explain.
Answer:
Sodium permutit, \(\ce{Na2Al2Si2O8.xH2O}\), exchanges its \(\ce{Na+}\) ions for \(\ce{Ca^2+}\) and \(\ce{Mg^2+}\) ions of hard water.
After long use most of the sodium ions are used up and the permutit becomes ineffective.
NaOH reacts with HCl to give sodium chloride (\(\ce{NaOH + HCl -> NaCl + H2O}\)). A concentrated solution of NaCl is passed through the exhausted permutit to regenerate it.
In simple words: Salt solution puts the sodium ions back into the used permutit.

Teacher's Note:
1. The solid compound is common salt.
2. Regeneration makes permutit reusable.

Question 13. Jini took a container completely filled with water with some ice cubes floating on it. He observed the container expecting that water to overflow after the ice melted. Can you predict what he would finally see?
Answer:
The water will not overflow. The floating ice displaces a volume of water equal to the volume of the water that the ice cubes become on melting. So the water level remains the same and the container stays filled up to the brim.
In simple words: Melted ice just fills the space that it was pushing aside.

Teacher's Note:
1. Floating ice displaces its own weight of water.
2. The level does not change.

Question 14. Why is ion exchange process preferred over permutit process for the purification of water?
Answer:
In the permutit process only the cations that cause hardness are removed.
In the ion exchange process both cations and anions are removed in the cation and anion exchangers. The water obtained is almost free of ions (demineralized water) and can be used in laboratories.
In simple words: Ion exchange removes all the ions, while permutit changes only calcium and magnesium into sodium.

Teacher's Note:
1. Mention both exchangers.
2. Demineralized water is much purer.

Question 15. In cold countries, water pipes burst in winter. Explain.
Answer:
In cold countries the temperature falls below 0°C. The water in the pipes freezes, and as water expands on freezing (ice has a larger volume), the ice exerts a huge pressure on the pipes and they burst.
In simple words: Water takes more space as ice, and the pipes cannot hold it.

Teacher's Note:
1. This is an effect of anomalous expansion.
2. Bottles left in a freezer crack for the same reason.

Question 16. Generally when a person jumps into water in any water body, he is expected to sink. But he tends to float in the Dead Sea. How do you account for this?
Answer:
The Dead Sea has a very large amount of dissolved salts, up to saturation, so the density of its water is high. The density of the human body is less than the density of the water of the Dead Sea, so the human body floats.
In simple words: The Dead Sea water is so salty and dense that it holds up a person.

Teacher's Note:
1. A body floats in a liquid denser than itself.
2. Salt water is denser than fresh water.

Question 17. During the conversion of surface water to potable water, chlorination is an important step. Give reasons.
Answer:
Chlorination is the addition of chlorine, which comes from bleaching powder. Chlorine reacts with water to form substances (such as hypochlorous acid, which can produce nascent oxygen) that kill the harmful germs and bacteria. Sedimentation and filtration do not remove all the microorganisms, so chlorination makes the water safe to drink.
In simple words: Chlorine kills the germs that filtering leaves behind.

Teacher's Note:
1. It is the last step of purification.
2. It stops water-borne diseases.

Question 18. Thermal pollution of water leads to the death of biotic life. Explain.
Answer:
Heavy industries such as steel plants and thermal power plants discharge large amounts of hot water into rivers. The warm water helps algae to grow rapidly, and the algae use up the dissolved oxygen in water. The fall in dissolved oxygen leads to the death of aquatic organisms.
In simple words: Hot water makes algae grow, the algae use up the oxygen and the fishes suffocate.

Teacher's Note:
1. Warm water also holds less oxygen.
2. Cooling towers reduce thermal pollution.