CBSE Class 7 Science Advanced Chemistry Water MCQs Set 02

Answer: (a) increase in the boiling point of water

Explanation:
1. In a closed cooker the pressure of steam rises above atmospheric pressure.
2. Higher pressure raises the boiling point of water, so the food cooks at a higher temperature in less time.
3. So the answer is increase in the boiling point of water.

Teacher's Note:
1. Higher pressure means a higher boiling point.
2. On a hill the opposite happens, and cooking is slow.

Answer: (d) introducing artificial fertilizers

Explanation:
1. Artificial fertilizers raise crop yield, which makes farming wealthy.
2. But the chemicals pollute ground water and food, so it is not healthy.
3. So the answer is introducing artificial fertilizers.

Teacher's Note:
1. Wealthy means a high yield, and not healthy means harm to the environment.
2. The other three options are eco-friendly.

Answer: (b) B

Explanation:
1. Steam at 100°C carries an extra 540 cal of latent heat in every gram.
2. When it condenses on the skin it gives out this heat along with the heat of the water.
3. So person B, burnt by steam, is affected more.

Teacher's Note:
1. Steam burns are worse than boiling water burns.
2. The difference is the latent heat of vaporization.

Answer: (d) both (a) and (c)

Explanation:
1. Ice is less dense than water, so it floats and forms a layer on top.
2. Ice is a poor conductor of heat, so this layer protects the water below from the cold air, and it stays liquid.
3. (b) is wrong because water is densest, not least dense, at 4°C.
4. So the answer is both (a) and (c).

Teacher's Note:
1. Ice on top works like a blanket.
2. The water under the ice stays near 4°C.

Answer: (c) chlorine

Explanation:
1. Chlorine, released from bleaching powder, kills harmful germs in water.
2. So the element is chlorine.

Teacher's Note:
1. The process is called chlorination.
2. Chlorine is added in small, safe amounts.

Answer: (c) cool breeze from air coolers

Explanation:
1. Bottles in ice cool because of the latent heat of fusion of ice, and hail cools the air for the same reason.
2. Pipes burst because water expands on freezing.
3. In an air cooler the wet mats take heat from the warm air, and this cools the breeze, which is the use of water as a heat absorber.
4. As in the book's key, the answer is cool breeze from air coolers.

Teacher's Note:
1. Eliminate the options that depend on ice.
2. Pipe bursting is about expansion, not specific heat.

Answer: (b) \(\ce{MgSO4}\), \(\ce{Na2CO3}\)

Explanation:
1. The products are \(\ce{MgCO3}\) (insoluble) and a sulphate of a monovalent metal, so X is \(\ce{MgSO4}\) and Y is a carbonate of sodium.
2. \(\ce{MgSO4 + Na2CO3 -> MgCO3 + Na2SO4}\).
3. So the answer is \(\ce{MgSO4}\) and \(\ce{Na2CO3}\).

Teacher's Note:
1. Work backwards from the products.
2. Sulphate in the products means sulphate in X.

Answer: (c) 1.5 g

Explanation:
1. Heat = mass × 1 cal g⁻¹ °C⁻¹ × rise in temperature.
2. 15 = X × 1 × 10, so X = 15 ÷ 10 = 1.5 g.

Teacher's Note:
1. Put the numbers into Q = m × s × t and solve for m.
2. Check by multiplying back: 1.5 × 10 = 15.

Answer: (a) devoid of dissolved salts and minerals

Explanation:
1. The cation and anion exchangers remove all positive and negative ions.
2. The water that comes out is demineralized water, free of dissolved salts and minerals.
3. It is not potable because it has no minerals, and it is not mineral water. So the answer is (a).

Teacher's Note:
1. Demineralized means minerals removed.
2. Pure is not the same as fit to drink.

Answer: (b) Cooking food at higher altitudes takes longer period of time.

Explanation:
1. At higher altitudes the air pressure is lower, so water boils at a lower temperature.
2. Food cooks more slowly in this cooler boiling water, so cooking takes longer.
3. The pressure cooker works on the opposite principle. So the answer is (b).

Teacher's Note:
1. Low pressure means a low boiling point.
2. The pressure cooker is the reverse case.

Answer: (b) washing soda

Explanation:
1. Washing soda changes the soluble chlorides and sulphates of calcium and magnesium into insoluble carbonates.
2. The carbonates settle as a precipitate, so the answer is washing soda.

Teacher's Note:
1. Washing soda is sodium carbonate.
2. It is the usual chemical for permanent hardness.

Answer: (d) using artificial fertilizers

Explanation:
1. Artificial fertilizers increase the yield, but pollute water and soil.
2. Biogas plants, neem oil and drip irrigation are good for the environment.
3. So the answer is using artificial fertilizers.

Teacher's Note:
1. Wealthy refers to high yield; not healthy refers to pollution.
2. Natural methods are healthy farming.

Answer: (b) A → b; B → e; C → a; D → d

Explanation:
1. Rain water is the purest form of natural water: A → b.
2. River and lake water is fresh water used for many activities: B → e.
3. Underground water is filtered by the soil, so it is free of suspended impurities: C → a.
4. Demineralized water is the water coming from an ion exchange apparatus: D → d.
5. This matches option (b).

Teacher's Note:
1. Match the sure pairs first and strike out wrong options.
2. Rain water is always the purest natural water.

Answer: (b) 20 cal

Explanation:
1. Specific heat of water = 1 cal g⁻¹ °C⁻¹.
2. Heat = 20 g × 1 × 1°C = 20 cal.

Teacher's Note:
1. Use Q = m × s × rise in temperature.
2. 1 cal raises 1 g by 1°C.

Answer: (c) 3 2 1 4 5

Explanation:
1. Water first evaporates (3).
2. The vapour condenses into droplets (2), and droplets join dust to form micelles (1).
3. Micelles gather as clouds (4), and rain falls (5).
4. The order is 3, 2, 1, 4, 5.

Teacher's Note:
1. Follow the water from the ground to the sky and back.
2. Micelles come before clouds.

Answer: (c) \(\ce{CaCl2}\), \(\ce{CaCO3}\)

Explanation:
1. Common salt (\(\ce{NaCl}\)) means X is a chloride.
2. Y is calcium carbonate, which gives \(\ce{CO2}\) with dilute acid.
3. \(\ce{CaCl2 + Na2CO3 -> CaCO3 + 2NaCl}\), so the answer is \(\ce{CaCl2}\) and \(\ce{CaCO3}\).

Teacher's Note:
1. Common salt in the products points to a chloride.
2. Calcium carbonate is the source of \(\ce{CO2}\) in the lab.

Answer: (c) 3 4 1 2

Explanation:
1. First the water is left to settle (3).
2. Alum and lime are then added (4), the water is filtered (1), and finally it is chlorinated (2).
3. The order is 3, 4, 1, 2.

Teacher's Note:
1. Big impurities go first, germs last.
2. Chemicals help the finer particles to settle.

Answer: (b) B

Explanation:
1. Steam at 100°C holds the latent heat of vaporization, 540 cal/g, in addition to the heat of boiling water.
2. This extra heat is released when steam condenses on the skin, so B is affected more.

Teacher's Note:
1. Steam burns are worse than hot water burns.
2. Think about the stored latent heat.

Answer: (c) oxygen, carbon dioxide

Explanation:
1. Drinking water has dissolved \(\ce{O2}\) and \(\ce{CO2}\).
2. So the answer is oxygen and carbon dioxide.

Teacher's Note:
1. These gases give water its taste.
2. Distilled water has none of them.

Answer: (a) hydrogen and oxygen

Explanation:
1. Water splits into hydrogen and oxygen in the volume ratio 2 : 1.
2. The tube with the double volume holds hydrogen, so A is hydrogen and B is oxygen.

Teacher's Note:
1. Double volume means hydrogen.
2. Always compare the volumes in the two tubes.

Answer: (c) usage of water as a coolant

Explanation:
1. Cooling by ice and by hail depends on latent heat, and bursting of pipes depends on expansion on freezing.
2. Using water as a coolant depends on its high specific heat.
3. So the answer is usage of water as a coolant.

Teacher's Note:
1. High specific heat means water absorbs a lot of heat for a small temperature rise.
2. Car radiators are an example.

Answer: (a) increase in the boiling point of water

Explanation:
1. Steam builds up pressure inside the closed cooker.
2. More pressure means a higher boiling point, so the food is cooked at a higher temperature and quicker.
3. So the answer is increase in the boiling point of water.

Teacher's Note:
1. Pressure and boiling point rise together.
2. Cooking time is reduced, not increased.

Answer: (b) increases

Explanation:
1. Ice is less dense than water, so a given mass of it takes more space.
2. So the volume increases.

Teacher's Note:
1. Ice floats on water because of the same reason.
2. Anomalous expansion is behind it.

Answer: (b) \(\ce{Ca(HCO3)2}\) and \(\ce{MgCl2}\)

Explanation:
1. Bicarbonates cause temporary hardness, while chlorides and sulphates of calcium and magnesium cause permanent hardness.
2. Option (a) has sodium sulphate, which does not cause hardness; (c) is in the wrong order; (d) has two permanent hardness salts.
3. So the answer is \(\ce{Ca(HCO3)2}\) and \(\ce{MgCl2}\).

Teacher's Note:
1. The first salt must be a bicarbonate and the second a chloride or sulphate.
2. Check the sodium compound in option (a).

Answer: (d) waste obtained from a biogas plant

Explanation:
1. Biodegradable substances are broken down by microorganisms.
2. Detergents, pesticides and polythene stay in the environment for a long time.
3. Waste from a biogas plant is organic and acts as manure, so the answer is waste obtained from a biogas plant.

Teacher's Note:
1. Organic waste is biodegradable.
2. Plastics are not.

Answer: (a) A → b; B → a; C → d, c; D → e

Explanation:
1. Distilled water is used in laboratory work: A → b.
2. Potable water is for drinking: B → a.
3. Spring water has dissolved salts and rare minerals and is used as medicine: C → c, d.
4. Acidulated water is used in electrolysis: D → e.
5. This matches option (a).

Teacher's Note:
1. Match what you are sure about first.
2. More than one entry in B can fit one entry in A.

Answer: (c) 1.5 g

Explanation:
1. Heat = mass × 1 cal g⁻¹ °C⁻¹ × 10°C, so 15 = X × 10.
2. X = 1.5 g.

Teacher's Note:
1. Rearrange Q = m × s × t to find m.
2. Check: 1.5 × 10 = 15 cal.

Answer: (b) 4 3 1 2 5

Explanation:
1. Vapour first falls as precipitation (4).
2. The water seeps through the zone of aeration (3), then the recharge area (1).
3. It reaches the water table (2), below which lies the aquifer (5).
4. The order is 4, 3, 1, 2, 5.

Teacher's Note:
1. Go down layer by layer from the surface.
2. The water table lies above the aquifer.

Answer: (b) \(\ce{MgSO4}\), \(\ce{Na2CO3}\)

Explanation:
1. The products are \(\ce{MgCO3}\) and a monovalent metal sulphate, so X is \(\ce{MgSO4}\) and Y is \(\ce{Na2CO3}\).
2. \(\ce{MgSO4 + Na2CO3 -> MgCO3 + Na2SO4}\).
3. So the answer is \(\ce{MgSO4}\) and \(\ce{Na2CO3}\).

Teacher's Note:
1. Use the products to find the reactants.
2. Sulphate must come from X.

Answer: (a) 3 1 2 4 5 6

Explanation:
1. Alum is fed first (3) and lime next (1), and then they are mixed (2).
2. The water goes to the sedimentation tank (4), through gravel and sand (5), and finally chlorine is added (6).
3. The order is 3, 1, 2, 4, 5, 6.

Teacher's Note:
1. Follow the labels of the purification plant figure from left to right.
2. Chlorine comes last.

Answer: (c) 800 cal, 5400 cal

Explanation:
1. Fusion: 10 g × 80 cal/g = 800 cal.
2. Vaporization: 10 g × 540 cal/g = 5400 cal.
3. So the answer is 800 cal, 5400 cal.

Teacher's Note:
1. Heat = mass × latent heat per gram.
2. Use 80 for ice melting and 540 for water boiling.

Answer: (b) distilled water

Explanation:
1. During distillation the water is boiled, which drives away the dissolved gases.
2. So the answer is distilled water.

Teacher's Note:
1. Boiling removes dissolved gases.
2. That is why distilled water tastes flat.

Answer: (d) 2 : 1 ratio of hydrogen and oxygen, respectively

Explanation:
1. Water is made of hydrogen and oxygen in the volume ratio 2 : 1.
2. So the answer is 2 : 1 (hydrogen : oxygen).
3. 1 : 8 is the ratio by mass.

Teacher's Note:
1. Volume ratio is 2 : 1 and mass ratio is 1 : 8.
2. Do not interchange them.

Answer: (d) high specific heat

Explanation:
1. The engine becomes very hot and must be cooled.
2. Water absorbs a lot of heat with a small rise in temperature because of its high specific heat.
3. So the answer is high specific heat.

Teacher's Note:
1. A good coolant has high specific heat.
2. Water is cheap and easily available.

Answer: (b) Cooking food at higher altitudes takes longer period of time.

Explanation:
1. At higher altitudes the pressure is low, so water boils at a lower temperature.
2. Food cooks slowly in this cooler water, so cooking takes longer.
3. So the answer is (b).

Teacher's Note:
1. Lower pressure means a lower boiling point.
2. The pressure cooker uses the reverse idea.

Answer: (c) increases up to 4°C and then decreases

Explanation:
1. Between 0°C and 4°C water contracts on heating, so its density increases.
2. After 4°C it expands, so its density decreases.
3. So the answer is increases up to 4°C and then decreases.

Teacher's Note:
1. Maximum density is at 4°C.
2. Density rises before 4°C and falls after it.

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

Explanation:
1. Boiling decomposes the bicarbonates of calcium and magnesium into insoluble carbonates, so R is true.
2. But chlorides and sulphates, which cause permanent hardness, are not removed by boiling, so A is false.
3. So the answer is A is false, but R is true.

Teacher's Note:
1. Boiling removes only temporary hardness.
2. Test each statement separately before comparing them.

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

Explanation:
1. Hydrogen is liberated at the cathode and oxygen at the anode, so A is false.
2. The gas at the cathode is hydrogen, which puts off the splinter with a pop, so R is true.
3. So the answer is A is false, but R is true.

Teacher's Note:
1. Remember: hydrogen at the cathode (negative), oxygen at the anode (positive).
2. The pop sound is the test for hydrogen.

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

Explanation:
1. Clark's method adds slaked lime to turn bicarbonates into insoluble carbonates, so R is true.
2. Bicarbonates cause temporary hardness, so the method removes temporary hardness, not permanent hardness. Hence A is false.
3. So the answer is A is false, but R is true.

Teacher's Note:
1. Bicarbonates mean temporary hardness.
2. Clark's method and boiling both treat temporary hardness.

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

Explanation:
1. Electrolysis of acidulated water gives hydrogen and oxygen, so A is true.
2. Positive hydrogen ions move to the negative electrode (cathode) and negative hydroxyl ions move to the positive electrode (anode), so R is false.
3. So the answer is A is true, but R is false.

Teacher's Note:
1. Opposite charges attract.
2. Cation goes to the cathode, anion goes to the anode.