Q1. Permanent hardness of water cannot be removed by
(a) Boiling
(b) Adding caustic soda
(c) Distillation
(d) Adding soda
Answer: (a) Boiling
Explanation:
Permanent hardness is caused by dissolved salts of calcium and magnesium, such as chlorides and sulphates, which do not precipitate on boiling. Unlike temporary hardness, which can be removed by heating, permanent hardness requires chemical treatment or distillation.
Q2. The density of sea water is highest as
(a) Depth increases and salinity increases
(b) Depth decreases and salinity increases
(c) Depth increases and salinity decreases
(d) Depth decreases and salinity decreases
Answer: (a) Depth increases and salinity increases
Explanation:
Sea water density depends on temperature, salinity, and pressure. As depth increases, temperature drops and pressure rises, while salinity may also increase due to evaporation and ocean currents. These combined factors lead to higher density at greater depths.
Q3. Municipal water in India is generally treated with
(a) Chlorine
(b) Potassium permanganate
(c) Sodium Chloride
(d) Sodium Carbonate
(UPSC Prelims 1980)
Answer: (a) Chlorine
Explanation: Chlorine is widely used to disinfect municipal water, killing pathogens and ensuring safe drinking water.
Q4. If water contracts on freezing, which of the following would happen ?
(a) Icebergs will completely float on the surface of water
(b) Ice would become more dense
(c) Lakes would freeze from top to bottom
(d) Rocks will crack due to freezing or internal water
(UPSC Prelims 1981)
Answer: (b) Ice would become more dense
Explanation: If water contracted on freezing, ice would be denser than liquid water and sink. However, in reality, water expands upon freezing, making ice less dense and allowing it to float. If it contracted, icebergs wouldn’t float, and lake freezing patterns would change.
Q5. When the ice floating in a glass of water melts, the level of water will
(a) Increase
(b) Decrease
(c) Remain constant
(d) First increase and then decrease
(UPSC Prelims 1981)
Answer: (c) Remain constant
Explanation: Floating ice displaces an amount of water equal to its weight. When it melts, it turns into the same volume of water it had displaced. Hence, the water level remains unchanged, demonstrating Archimedes’ principle.
Q6. What happens when the temperature of water falls from 30°C to 0°C?
(a) The volume of ice formed will be less than volume of water
(b) The volume of ice formed will be more than the volume of water
(c) Water will not freeze at 0°C
(d) None of the above
(UPSC Prelims 1981)
Answer: (b) The volume of ice formed will be more than the volume of water
Explanation: As water cools to 0°C, it expands upon freezing, forming ice with a greater volume than the original water. This is due to the crystalline structure of ice, which occupies more space than liquid water.
Q7. When the rice is boiled, it expands because
(a) The starch gelatinises and swells
(b) The starch gets polymerised
(c) The grains absorb moisture and swell
(d) Sugars break down in the grains
(UPSC Prelims 1981)
Answer: (c) The grains absorb moisture and swell
Explanation: During boiling, rice grains absorb water, causing them to swell and expand. This is a physical change, where moisture penetrates the starch, making the grains soft and larger.
Q8. When a ship enters a sea from a river, its portion under water will
(a) Increase
(b) Decrease
(c) Show no change
(d) Increase and decrease alternately
(UPSC Prelims 1982)
Answer: (b)
Explanation: When a ship moves from a river (freshwater) to the sea (saltwater), the density of water increases, causing the ship to displace less water. As a result, the portion of the ship submerged decreases, demonstrating the principle of buoyancy.
Q9. The purest form of water is obtained from
(a) A deep tube well
(b) A running stream
(c) Hot water spring
(d) Heavy rains
(UPSC Prelims 1982)
Answer: (d)
Explanation: Rainwater, especially during initial showers, is considered the purest form as it results from natural distillation. It is free from dissolved salts, though it may pick up impurities from the atmosphere.
Q10. Why do you feel chill after having a dip in sea water ?
(a) The difference in temperature between sea water and the atmosphere
(b) Evaporation of water from the body
(c) Deposition of salt on the body
(d) None of these
(UPSC Prelims 1984)
Answer: (b) Evaporation of water from the body
Explanation: After a dip in sea water, evaporation occurs from the skin surface. This process absorbs heat from the body, causing a cooling effect. It is a physical phenomenon where latent heat of vaporization leads to a drop in body temperature, making one feel chilled.
Q11. The surface of the water in a lake is frozen and its temperature is -15℃. The maximum possible temperature of the water inside the lake in contact with the ice layer will be
(a) 0℃
(b) 4℃
(c) 15℃
(d) Insufficient data to predict
(UPSC Prelims 1984)
Answer: (b) 4℃
Explanation: Water has maximum density at 4℃, so in a frozen lake, the bottom layers remain at 4℃, while the surface can freeze at 0℃ or below. This thermal stratification helps aquatic life survive in winter.
Q12. When a brick is dropped in water, bubbles are seen to rise. This is due to
(a) Air in the pores of the brick
(b) Air in water
(c) Water displaced by the brick
(d) None of these
(UPSC Prelims 1984)
Answer: (a) Air in the pores of the brick
Explanation: Bricks are porous materials, meaning they contain tiny air pockets. When a brick is submerged in water, the air trapped in these pores escapes, forming bubbles that rise to the surface. This is a physical release of air, not a chemical reaction.
Q13. A small wooden block is floating in a tub of water. The water is gradually heated. The volume of the wooden block visible above the water level
(a) Increases
(b) Decreases
(c) Fluctuates
(d) Remains the same
(UPSC Prelims 1986)
Answer: (b) Decreases
Explanation: As water is heated, its density decreases, causing the buoyant force to reduce. Consequently, the wooden block sinks slightly, and the visible volume above water decreases.
Q14. If a toy boat in a tank sinks, the level of water will
(a) Increase
(b) Decrease
(c) Fluctuate
(d) Remain the same
(UPSC Prelims 1986)
Answer: (a) Increase
Explanation: When the toy boat sinks, it displaces more water than when it was floating. This leads to an increase in water level, due to the principle of displacement.
Q15. Of the total water on the earth, fresh water reserves constitute approximately
(a) 1.2 %
(b) 2.7 %
(c) 4.5 %
(d) 5.8 %
(UPSC Prelims 1987)
Answer: (b) 2.7 %
Explanation: Only about 2.7% of Earth’s water is freshwater, and most of it is locked in glaciers and ice caps. The remaining is found in rivers, lakes, and groundwater, making it a scarce and vital resource.