CBSE Class 7 Science Advanced Chemistry Water MCQs Set 04

Answer: (b) anomalous expansion

Explanation:
1. Most liquids expand steadily on heating, but water contracts from 0°C to 4°C and expands only after that.
2. This unusual behaviour is called anomalous expansion.

Teacher's Note:
1. Anomalous means unusual.
2. This protects aquatic life in freezing lakes.

Answer: (c) convert 1 g of water at 100°C to steam at 100°C

Explanation:
1. Latent heat of vaporization is the heat needed to change a liquid into vapour at its boiling point without a rise in temperature.
2. Converting 1 g of water at 100°C to steam at 100°C fits this. Option (a) is specific heat and (b) is latent heat of fusion.
3. The answer is (c).

Teacher's Note:
1. Vaporization means liquid to gas at the same temperature.
2. Option (d) has the steam at 10°C, which is not at the boiling point.

Answer: (d) Boiling point of water is 100°C at any pressure.

Explanation:
1. Statements (a), (b) and (c) are correct.
2. The boiling point of water changes with pressure; it is 100°C only at normal atmospheric pressure.
3. So the incorrect statement is (d).

Teacher's Note:
1. The word 'incorrect' means you hunt for the false one.
2. Water boils at a lower temperature on a mountain.

Answer: (c) BACED

Explanation:
1. Rain and river water seep into the soil: (B).
2. This seepage is infiltration and gives underground water: (A).
3. The water passes through the zone of aeration to the zone of saturation: (C).
4. At the top of the saturated zone is the water table: (E).
5. The water-bearing rock below is the aquifer: (D).
6. The order is B, A, C, E, D: option (c).

Teacher's Note:
1. Follow the water from the surface downward.
2. The water table comes before the aquifer in the path from the top.

Answer: (b) temporary hard water

Explanation:
1. Bicarbonates of calcium and magnesium cause temporary hardness, because boiling removes them.
2. The water is temporary hard water.

Teacher's Note:
1. Bicarbonates mean temporary hardness.
2. Chlorides and sulphates mean permanent hardness.

Answer: (c) Rain water harvesting

Explanation:
1. Drought (the book spells it 'draught'), deforestation and a growing population all reduce the water supply.
2. Rain water harvesting stores water and adds to it, so it is not a cause of depletion.
3. The answer is (c).

Teacher's Note:
1. Pick the activity that saves or adds water.
2. More people means more demand.

Answer: (b) (i) → (E); (ii) → (A); (iii) → (B); (iv) → (D)

Explanation:
1. Scum forms when soap is used with hard water: (i) → (E).
2. Hydrated sodium aluminium silicate is permutit: (ii) → (A).
3. Calcium chloride causes permanent hardness: (iii) → (B).
4. Desalinated water is obtained by reverse osmosis: (iv) → (D).
5. This is option (b).

Teacher's Note:
1. Scum is the curdy precipitate soap forms with Ca and Mg ions.
2. Reverse osmosis removes salts from sea water.

Answer: (d) BADCE

Explanation:
1. The tubes are first filled with acidulated water: (B).
2. They are inverted over the iron nails joined to the battery: (A).
3. The nails act as electrodes: (D).
4. Current is passed and gases collect: (C).
5. The gases are tested and identified: (E).
6. The order is B, A, D, C, E: option (d).

Teacher's Note:
1. Set up first, run the experiment, then test the gases.
2. Oxygen and hydrogen are in the ratio 1:2 by volume.

Answer: (c) \(\ce{CaCl2 + Na2CO3 -> CaCO3 + 2NaCl}\)

Explanation:
1. Temporary hardness is due to bicarbonates, so reactions of bicarbonates, by heating or with lime, remove it: (a), (b) and (d).
2. Option (c) involves calcium chloride, which causes permanent hardness.
3. So (c) is not involved in removing temporary hardness: (c).

Teacher's Note:
1. Look for the equation without a bicarbonate.
2. Chloride means permanent hardness.

Answer: (c) Hot water.

Explanation:
1. Thermal power plants use water for cooling and discharge it into reservoirs while it is still hot.
2. The pollutant is hot water, and it reduces the dissolved oxygen.

Teacher's Note:
1. Think of what leaves the cooling system.
2. Thermal pollution harms fish.

Answer: (b) Salt and minerals

Explanation:
1. Water of crystallization is water held inside the crystals of salts and minerals, such as \(\ce{CuSO4.5H2O}\).
2. Ice is simply solid water and an aqueous solution is a dissolved state, so neither is water of crystallization.
3. The answer is salt and minerals.

Teacher's Note:
1. Water of crystallization is part of the crystal structure.
2. Blue vitriol is an example.

Answer: (a) proportional to external pressure

Explanation:
1. A liquid boils when its vapour pressure equals the external pressure.
2. Higher external pressure raises the boiling point, as in a pressure cooker, so it increases with external pressure.
3. The answer is (a).

Teacher's Note:
1. Pressure cookers cook faster as water boils above 100°C inside.
2. At high altitude the boiling point falls.

Answer: (c) A is true, but R is false.

Explanation:
1. A is true: water dissolves a large number of substances, so it is called the universal solvent.
2. R is false: pure water has no ions, so it is a poor conductor. Only water with dissolved ions conducts electricity.
3. So A is true and R is false: (c).

Teacher's Note:
1. Check each statement alone.
2. Distilled water does not conduct; salty water does.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A is true: a large amount of heat is needed to raise the temperature of water by 1°C.
2. R is true: water shows anomalous expansion, with density rising from 0°C to 4°C.
3. But the density change has nothing to do with specific heat, so R is not the explanation: (b).

Teacher's Note:
1. Ask whether R gives the reason behind A.
2. Specific heat depends on how water stores heat; density anomaly depends on its structure.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: clouds, fog and mist are tiny droplets formed when water vapour condenses.
2. R is true: the atmosphere contains water vapour.
3. Condensation of this vapour gives clouds, fog and mist, so R explains A: (a).

Teacher's Note:
1. Ask where the droplets come from.
2. They come from water vapour in the air.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A is true: 540 cal of heat changes 1 g of water at 100°C into steam at 100°C.
2. R is true: water has a high specific heat, so it needs more heat to warm up than most liquids.
3. But the latent heat value is not explained by the specific heat, so R is not the explanation: (b).

Teacher's Note:
1. Latent heat is for change of state; specific heat is for change in temperature.
2. Both can be true without one explaining the other.

Answer: (d) A is false, but R is true.

Explanation:
1. A is false: temporary hardness is caused by bicarbonates of calcium and magnesium. Chlorides cause permanent hardness.
2. R is true: boiling decomposes the bicarbonates into insoluble carbonates.
3. So A is false and R is true: (d).

Teacher's Note:
1. Chlorides and sulphates mean permanent hardness.
2. Bicarbonates mean temporary hardness.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A is true: the resin is an insoluble porous solid, so it stays in the column.
2. R is true: the name comes from the ions it can exchange.
3. But the mobile ions explain the name, not why the resin is insoluble, so R is not the explanation: (b).

Teacher's Note:
1. Ask if R explains insolubility.
2. R explains the name 'exchange resin' only.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A is true: permutit is hydrated sodium aluminium silicate.
2. R is true: the sodium ions of permutit replace the Ca²⁺ and Mg²⁺ ions.
3. But R describes how permutit works, not why it is called permutit, so R is not the explanation: (b).

Teacher's Note:
1. A gives the name and R gives the function.
2. Both can be true without one explaining the other.

Answer: (c) A is true, but R is false.

Explanation:
1. A is true: the ion exchange apparatus removes the mineral ions, so the water that comes out is demineralized water.
2. R is false: demineralized water has no mineral ions.
3. So A is true and R is false: (c).

Teacher's Note:
1. The prefix 'de' means removed.
2. Check what the word demineralized means.