CBSE Class 7 Science Advanced Chemistry Water MCQs Set 01

Answer: (d) all the above

Explanation:
1. Fertilizers, insecticides and pesticides used in farming get washed into rivers and seep into the ground.
2. All three are pollutants of water, so the answer is all the above.

Teacher's Note:
1. Farm chemicals are a major source of water pollution.
2. When two or three options are right, look for the 'all the above' choice.

Answer: (a) 76 cm of Hg

Explanation:
1. Water freezes at 0°C under normal atmospheric pressure.
2. Normal atmospheric pressure is 76 cm of Hg (760 mm of Hg).
3. The other options mix up the units.

Teacher's Note:
1. Remember 76 cm = 760 mm of mercury.
2. Both the freezing point and the boiling point are quoted at this pressure.

Answer: (b) mercury, cadmium

Explanation:
1. Mercury and cadmium salts are poisonous and come from industries.
2. Calcium and aluminium salts are not toxic in this way.
3. So the pair is mercury and cadmium.

Teacher's Note:
1. Poisonous heavy metals such as mercury, cadmium, chromium and arsenic are industrial pollutants.
2. Calcium is a common mineral in hard water, not a toxic pollutant.

Answer: (a) rain water, sea water

Explanation:
1. Rain water is the purest natural water, and sea water has a lot of dissolved salt.
2. (b) has two pure kinds, (c) and (d) have two impure kinds.
3. So the pair (pure, impure) is rain water, sea water.

Teacher's Note:
1. The first item of the pair must be pure and the second impure.
2. Rain water is the purest form of natural water.

Answer: (a) bleaching powder

Explanation:
1. Bleaching powder gives out chlorine, which kills harmful bacteria.
2. Alum only helps suspended impurities to settle, and oxygen and carbon dioxide do not kill germs.
3. So the answer is bleaching powder.

Teacher's Note:
1. Killing germs is called disinfection, and chlorine does it.
2. Alum is used for settling, not for killing bacteria.

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

Explanation:
1. Water has its highest density, 1 g/cc, at 4°C, so at 4°C it is not less than 1 g/cc.
2. Above 4°C the density falls below 1 g/cc, and below 4°C it also falls below 1 g/cc.
3. So the answer is both (b) and (c).

Teacher's Note:
1. Draw a small curve of density against temperature with its peak at 4°C.
2. Both sides of the peak lie lower.

Answer: (b) 540 cal/g

Explanation:
1. 1 g of water at 100°C needs 540 cal to change into steam at the same temperature.
2. 80 cal/g is the latent heat of fusion of ice.
3. So the answer is 540 cal/g.

Teacher's Note:
1. Learn the pair: 80 cal/g to melt ice and 540 cal/g to boil water.
2. Vaporization always needs more heat than fusion.

Answer: (b) addition of washing soda

Explanation:
1. Clark's method and boiling remove only temporary hardness.
2. Washing soda reacts with bicarbonates, chlorides and sulphates of calcium and magnesium, so it removes both kinds.
3. So the answer is addition of washing soda.

Teacher's Note:
1. Washing soda is the all-round softener in this list.
2. Do not choose 'all the above' unless every option fits.

Answer: (d) potable water

Explanation:
1. Infiltration, aquifer and water table are all parts of the story of underground water.
2. Potable water means water fit to drink, which can come from any source.
3. So the odd term is potable water.

Teacher's Note:
1. Infiltration fills, the water table marks the top and the aquifer holds the water.
2. Potable describes quality, not location.

Answer: (c) potable water

Explanation:
1. Potable water has no suspended impurities or germs and keeps useful salts and gases.
2. Demineralized and distilled water lack minerals and dissolved gases, and soft water may be impure.
3. So the answer is potable water.

Teacher's Note:
1. Potable simply means 'fit to drink'.
2. Pure is not always the same as healthy to drink.

Answer: (b) increases

Explanation:
1. Ice is less dense than water, so the same mass of ice takes up more space.
2. So the volume increases.

Teacher's Note:
1. Ice floats because it is less dense.
2. This is why a full bottle of water cracks in a freezer.

Answer: (c) oxygen, carbon dioxide

Explanation:
1. Potable water contains dissolved oxygen and carbon dioxide.
2. Nitric oxide and carbon monoxide are not normally present, and the book does not list nitrogen.
3. So the answer is oxygen and carbon dioxide.

Teacher's Note:
1. These two gases give water its fresh taste.
2. Distilled water loses them during distillation.

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

Explanation:
1. Bicarbonates of calcium and magnesium cause temporary hardness.
2. Chlorides and sulphates of calcium and magnesium cause permanent hardness.
3. \(\ce{Na2SO4}\) (sodium sulphate) does not cause hardness, so (a) is wrong.
4. In (b) the first salt is a bicarbonate and the second is a chloride, so (b) is correct.

Teacher's Note:
1. Bicarbonate means temporary, chloride or sulphate means permanent.
2. Sodium salts do not make water hard.

Answer: (c) 52314

Explanation:
1. The source is the lake (5).
2. Then the water settles in a tank (2), is filtered (3), is chlorinated (1) and is finally stored (4).
3. The order is 5, 2, 3, 1, 4.

Teacher's Note:
1. Start with the source and end with the storage tank.
2. Chlorination comes after filtration, because it kills germs in already clear water.

Answer: (b) 32145

Explanation:
1. Water in water bodies evaporates to form water vapour (3).
2. The vapour condenses into clouds (2) and falls as precipitation (1).
3. The water then seeps into the soil (4) and reaches the ground water table (5).
4. The order is 3, 2, 1, 4, 5.

Teacher's Note:
1. Follow the drop of water on its journey.
2. Evaporation always starts the cycle from water bodies.

Answer: (d) distillation

Explanation:
1. Sedimentation, filtration and chlorination are the steps used in a water treatment plant.
2. Distillation is used to make pure water for laboratories, not for drinking water supply.
3. So the answer is distillation.

Teacher's Note:
1. Distillation removes everything, including useful salts.
2. Town water supply uses settling, filtering and chlorine.

Answer: (b) sea water

Explanation:
1. Sea water has a lot of dissolved salt, about 2.5% of it common salt.
2. Rain, spring and river water have far less dissolved salt.
3. So the answer is sea water.

Teacher's Note:
1. Saline water is not fit for drinking without desalination.
2. Reverse osmosis can turn sea water into fresh water.

Answer: (b) at 4°C

Explanation:
1. Because of anomalous expansion, water contracts on warming from 0°C to 4°C and expands afterwards.
2. So its density is highest, 1 g/cc, at 4°C.

Teacher's Note:
1. Maximum density means minimum volume.
2. This is the one temperature that is a peak for water.

Answer: (a) manures

Explanation:
1. Pesticides, fertilizers and insecticides are chemicals that wash into water and pollute it.
2. Manure is organic matter that bacteria can break down, so it causes much less pollution.
3. So the answer is manures.

Teacher's Note:
1. Natural manure is recommended instead of artificial fertilizers.
2. Biodegradable means it can be broken down by microorganisms.

Answer: (b) Na and Al

Explanation:
1. Permutit is hydrated sodium aluminium silicate, \(\ce{Na2Al2Si2O8.xH2O}\).
2. The metals in it are sodium (Na) and aluminium (Al).
3. So the answer is Na and Al.

Teacher's Note:
1. The name sodium aluminium silicate gives away the two metals.
2. Potassium is not a part of permutit.

Answer: (d) water

Explanation:
1. Water has a specific heat of 1 cal g⁻¹ °C⁻¹, the highest among common substances.
2. Petrol, mercury and oil all have much smaller values.
3. So the answer is water.

Teacher's Note:
1. High specific heat makes water a good coolant.
2. Mercury heats up very quickly because its specific heat is low.

Answer: (d) distilled water

Explanation:
1. Dissolved salts make saline, tap and river water good conductors.
2. Distilled water has no dissolved salts, so it is a poor conductor.
3. So the answer is distilled water.

Teacher's Note:
1. Ions carry the current in water.
2. No dissolved salts means no ions.

Answer: (c) usage of fertilizers

Explanation:
1. Natural manure, drip irrigation and natural pesticides all reduce water pollution.
2. Artificial fertilizers add to water pollution.
3. So the answer is usage of fertilizers.

Teacher's Note:
1. Read 'not' carefully in the question.
2. Natural options are the control measures.

Answer: (d) nitrate, phosphate

Explanation:
1. Bicarbonate, sulphate and chloride salts of calcium and magnesium cause hardness.
2. Nitrate and phosphate do not.
3. So the answer is nitrate, phosphate.

Teacher's Note:
1. Learn the three hardness-causing ions: bicarbonate, chloride and sulphate.
2. Everything else is not a cause of hardness.

Answer: (a) sodium hydroxide solution

Explanation:
1. The anion exchange resin gives out \(\ce{OH-}\) ions, and when it runs out of them it needs to be refilled with \(\ce{OH-}\).
2. Sodium hydroxide solution supplies the hydroxide ions.
3. So the answer is sodium hydroxide solution.

Teacher's Note:
1. Regenerate the resin with the ion it originally held.
2. The cation resin uses \(\ce{HCl}\) or \(\ce{NaCl}\), the anion resin uses \(\ce{NaOH}\).

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

Explanation:
1. Spring water has been filtered through the ground, so it has no suspended impurities.
2. It does contain dissolved salts and minerals.
3. So the answer is both (a) and (b).

Teacher's Note:
1. Underground water means no suspended impurities.
2. Dissolved salts and minerals make it useful for medicine.

Answer: (b) distilled water

Explanation:
1. Dissolved gases are driven out when water is boiled during distillation.
2. Potable, saline and soft water still contain them.
3. So the answer is distilled water.

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

Answer: (b) washing soda

Explanation:
1. Washing soda (sodium carbonate) gives insoluble carbonates of calcium and magnesium.
2. These settle down as a precipitate and are filtered.
3. So the answer is washing soda.

Teacher's Note:
1. Precipitation reaction means an insoluble solid forms.
2. Baking soda and alum do not do this job.

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

Explanation:
1. From 0°C to 4°C the density of water rises, because water contracts.
2. After 4°C the density falls as the water expands.
3. So the density increases up to 4°C and then decreases.

Teacher's Note:
1. Picture the density curve with its peak at 4°C.
2. The peak is the key to this question.

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

Explanation:
1. From \(\ce{2H2 + O2 -> 2H2O}\), two volumes of hydrogen combine with one volume of oxygen.
2. So the ratio by volume is 2 : 1 for hydrogen and oxygen respectively.
3. The ratio 1 : 8 is the ratio by mass, not by volume.

Teacher's Note:
1. Volume ratio follows the coefficients in the equation.
2. Do not mix up the volume ratio (2 : 1) with the mass ratio (1 : 8).

Answer: (a) 1324

Explanation:
1. Alum and lime are fed in first (1).
2. They are mixed with mechanical blades (3), and the water is then filtered (2).
3. Chlorination (4) comes last to kill germs.
4. The order is 1, 3, 2, 4.

Teacher's Note:
1. Chemicals go in, get mixed, then solids are filtered out.
2. Disinfecting is always the last step.

Answer: (a) 13245

Explanation:
1. Ice has less heat content than water, and water has less than steam of the same mass.
2. Among the ice samples, the colder one at −40°C has less heat than the one at −10°C, so (1) comes before (3).
3. Among water samples, 10°C has less heat than 40°C, so (2) comes before (4). Steam (5) has the most.
4. The order is 1, 3, 2, 4, 5.

Teacher's Note:
1. Higher temperature means more heat within the same state.
2. Changing state from ice to water to steam adds a large amount of heat.

Answer: (c) 800 cal, 5400 cal

Explanation:
1. Latent heat of fusion of ice = 80 cal/g, so for 10 g it is 10 × 80 = 800 cal.
2. Latent heat of vaporization of water = 540 cal/g, so for 10 g it is 10 × 540 = 5400 cal.
3. So the answer is 800 cal, 5400 cal.

Teacher's Note:
1. Multiply the mass by the latent heat per gram.
2. Fusion is 80 cal/g and vaporization is 540 cal/g.

Answer: (c) 9/10 of iceberg submerged below sea water and 1/10 above the sea water

Explanation:
1. Ice is less dense than sea water, so an iceberg floats, which rules out (b).
2. About 9/10 of the iceberg stays under water and only 1/10 is seen above.
3. The hidden huge part could not be seen, so the ship hit it and sank.
4. So the answer is (c).

Teacher's Note:
1. Floating ice shows only its tip.
2. Remember the rule: 9/10 below, 1/10 above.

Answer: (d) distillation

Explanation:
1. Sedimentation, chlorination and filtration are used for making river and lake water potable.
2. Distillation is too costly for large supplies and removes useful salts.
3. So the answer is distillation.

Teacher's Note:
1. Town water is not distilled.
2. Distilled water is prepared for laboratory use.

Answer: (d) high specific heat

Explanation:
1. The engine produces a lot of heat that must be carried away.
2. Water has a very high specific heat, so it can absorb a lot of heat without becoming very hot.
3. So the property used is high specific heat.

Teacher's Note:
1. A good coolant has a high specific heat.
2. This is why water is also used in hot water bags.

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

Explanation:
1. Common salt is \(\ce{NaCl}\), so X must be a chloride, and X + \(\ce{Na2CO3}\) gives Y + \(\ce{NaCl}\).
2. Y is used to prepare \(\ce{CO2}\) in the laboratory, which is calcium carbonate (it reacts with dilute \(\ce{HCl}\)).
3. So X is \(\ce{CaCl2}\): \(\ce{CaCl2 + Na2CO3 -> CaCO3 + 2NaCl}\).
4. The answer is \(\ce{CaCl2}\) and \(\ce{CaCO3}\).

Teacher's Note:
1. Common salt tells you X is a chloride.
2. Marble (calcium carbonate) is the usual source of \(\ce{CO2}\) in the lab.

Answer: (b) 20 cal

Explanation:
1. The specific heat of water is 1 cal g⁻¹ °C⁻¹.
2. Heat = mass × specific heat × rise in temperature = 20 × 1 × 1 = 20 cal.

Teacher's Note:
1. Heat = m × s × change in temperature.
2. For water, s = 1, so the heat in calories equals mass × rise.

Answer: (a) hydrogen and oxygen

Explanation:
1. Electrolysis gives hydrogen and oxygen in the volume ratio 2 : 1.
2. The gas with double the volume is hydrogen, so A holds hydrogen and B holds oxygen.

Teacher's Note:
1. The larger volume is always hydrogen.
2. Read which test tube holds the bigger volume.

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

Explanation:
1. Distilled water and demineralized (ion exchange) water have no dissolved salts, so they conduct poorly.
2. Mineral water has dissolved salts, so it conducts well.
3. So the answer is both (a) and (b).

Teacher's Note:
1. Ions in water carry the current.
2. Mineral water has minerals, so it conducts.