CBSE Class 7 Science Advanced Chemistry Water MCQs Set 03

Answer: (b) Calcium hydroxide

Explanation:
1. Calcium oxide dissolves in water to form calcium hydroxide: \(\ce{CaO + H2O -> Ca(OH)2}\).
2. It has a divalent cation \(\ce{Ca^2+}\) and a monovalent anion \(\ce{OH-}\), and it is used to remove temporary hardness (Clark's method).
3. The carbonates are not bases formed by dissolving an oxide in water and are insoluble. The answer is calcium hydroxide.

Teacher's Note:
1. Match the clues: divalent cation, monovalent anion, formed from an oxide plus water.
2. Slaked lime is the common name of this base.

Answer: (c) \(\ce{Ca^2+}\) and \(\ce{Mg^2+}\)

Explanation:
1. The cation exchanger removes positive ions, and the ions responsible for hardness are calcium and magnesium.
2. So it exchanges the \(\ce{Ca^2+}\) and \(\ce{Mg^2+}\) ions of hard water for hydrogen ions.
3. Anions such as sulphate, chloride and bicarbonate are handled by the anion exchanger. The answer is (c).

Teacher's Note:
1. Cation exchanger means it deals with cations.
2. The anion exchanger replaces anions with OH⁻ ions.

Answer: (b) rain water harvesting

Explanation:
1. Deforestation reduces seepage, industrialization increases water use, and scanty rainfall reduces the supply, so all of these lead to shortage.
2. Rain water harvesting stores water and recharges the underground supply, so it does not cause shortage.
3. The answer is rain water harvesting.

Teacher's Note:
1. Look for the activity that adds water instead of using it up.
2. Harvesting rain raises the water table.

Answer: (d) DEACB

Explanation:
1. Demineralized water has no mineral ions: (D).
2. Rain water is the purest natural water: (E).
3. Potable water is fit for drinking: (A).
4. Saline water has dissolved salts: (C).
5. An aquifer is underground water reachable with a well: (B).
6. The order is D, E, A, C, B: option (d).

Teacher's Note:
1. Match one by one and note the letters in the order of the list of waters.
2. Rain water comes straight from the atmosphere.

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

Explanation:
1. Wastes from atomic reactors affect marine life: (i) → (E).
2. Industrial effluents destroy aquatic organisms: (ii) → (C).
3. Hot water from thermal power plants lowers dissolved oxygen: (iii) → (D).
4. Excreta spread water-borne diseases: (iv) → (A).
5. Fertilizers and pesticides from farms seep underground: (v) → (B).
6. This is option (b).

Teacher's Note:
1. Warm water holds less oxygen, which links thermal plants to (D).
2. Excreta and disease form the surest pair.

Answer: (a) Cetyl alcohol

Explanation:
1. Cetyl alcohol forms a thin film on the surface of the water, which cuts down evaporation.
2. Bleaching powder disinfects water, DDT is a pesticide and ethyl alcohol evaporates itself.
3. The answer is cetyl alcohol.

Teacher's Note:
1. Think of a thin layer that covers the water surface.
2. This is a water-saving measure in dry regions.

Answer: (d) CEBAD

Explanation:
1. Alum and lime are added first to make the suspended particles clump together: (C).
2. The mixture is stirred mechanically: (E).
3. The clumps settle in the sedimentation tank: (B).
4. The water is filtered through sand and gravel: (A).
5. Finally bleaching powder releases chlorine to kill bacteria: (D).
6. The order is C, E, B, A, D: option (d).

Teacher's Note:
1. Disinfection with chlorine is always the last step.
2. Chemicals are added before the particles can settle.

Answer: (a) Permutit, \(\ce{Na+}\) ion, permanent hardness

Explanation:
1. Permutit is hydrated sodium aluminium silicate. Its sodium ions replace the \(\ce{Ca^2+}\) and \(\ce{Mg^2+}\) ions of hard water, and the hardness is removed.
2. The other options name chemicals or ions that do not act by exchange: sodium carbonate and calcium hydroxide precipitate the ions, and sodium chloride is not an exchanger.
3. The answer is option (a).

Teacher's Note:
1. Exchange of ions by a solid chemical is the permutit process.
2. Na⁺ ions go into the water and Ca²⁺, Mg²⁺ stay behind in the permutit.

Answer: (b) hydrogen and oxygen

Explanation:
1. The gas that burns with a pop and a blue flame is hydrogen, and it collects at the negative electrode (cathode).
2. The gas that makes a glowing splinter burn brightly is oxygen, and it collects at the positive electrode (anode).
3. So the gases are hydrogen and oxygen.

Teacher's Note:
1. Hydrogen is the combustible gas and comes at the cathode.
2. Water is made of only hydrogen and oxygen.

Answer: (a) Demineralized water

Explanation:
1. Demineralized water has had all its mineral ions removed, but our body needs some minerals from drinking water.
2. Desalinated water, potable water and rain water contain at least some useful ions or are safe to drink.
3. The answer is demineralized water.

Teacher's Note:
1. Pure water without minerals does not supply the ions the body needs.
2. Potable means fit to drink.

Answer: (c) Washing soda

Explanation:
1. Washing soda (sodium carbonate) is added to the water and it precipitates the calcium and magnesium of both temporary and permanent hardness as carbonates.
2. Slaked lime removes only temporary hardness. Permutit and ion exchange resins are not added to the water; the water is passed through them.
3. The answer is washing soda.

Teacher's Note:
1. The question says 'added', which fits a chemical dissolved in the water.
2. Slaked lime works only for bicarbonates.

Answer: (d) Drip irrigation

Explanation:
1. Natural pesticides and fertilizers and biogas reduce pollution, but they do not directly cut the quantity of water used.
2. Drip irrigation delivers water drop by drop near the roots, so very little is wasted.
3. The answer is drip irrigation.

Teacher's Note:
1. Ask which practice changes how much water is applied.
2. Drip irrigation is common in dry regions.

Answer: (c) Respiration

Explanation:
1. Photosynthesis uses water as a raw material, sweating uses it to control body temperature, and water carries minerals and nutrients in plants and animals.
2. Respiration (the release of energy from food using oxygen) is the one among these that is not treated as depending on water as a medium.
3. The answer is respiration.

Teacher's Note:
1. Check each process for a role of water as a raw material or a carrier.
2. Respiration releases energy from food using oxygen.

Answer: (b) \(\ce{Ca^2+}\), \(\ce{Mg^2+}\)/\(\ce{Cl-}\), \(\ce{SO4^2-}\)

Explanation:
1. Hardness is caused by calcium and magnesium ions with bicarbonate, chloride or sulphate ions.
2. Sodium ions do not cause hardness, so (a) and (c) are wrong. Carbonates of calcium and magnesium are insoluble, so (d) is wrong.
3. The set \(\ce{Ca^2+}\), \(\ce{Mg^2+}\) with \(\ce{Cl-}\), \(\ce{SO4^2-}\) is option (b).

Teacher's Note:
1. Cross out any set that contains Na⁺.
2. Dissolved salts must be soluble to cause hardness.

Answer: (c) NaOH

Explanation:
1. An anion exchanger holds hydroxide ions, which it gives up in exchange for anions of the water.
2. To renew it, the resin is treated with a base, so sodium hydroxide solution is used.
3. The answer is NaOH.

Teacher's Note:
1. The anion exchanger must get its OH⁻ back.
2. Cation exchangers are regenerated with acid.

Answer: (d) Photosynthesis

Explanation:
1. Deforestation reduces seepage of rain water, industrialization increases use of water and scanty rainfall reduces recharge, so all lower the water table.
2. Photosynthesis is a natural process and does not draw down the underground water.
3. The answer is photosynthesis.

Teacher's Note:
1. Look for a natural process, not a human activity.
2. The water table is the level of underground water.

Answer: (c) BCAED

Explanation:
1. Industrialization uses more water: (B).
2. Construction covers the ground and reduces seepage: (C).
3. Deforestation lets rain water run off and erode the soil, so less seeps in: (A).
4. Scanty rainfall means almost no replenishment while water is still drawn out: (E).
5. Irrigation draws a lot of ground water: (D).
6. The order is B, C, A, E, D: option (c).

Teacher's Note:
1. The causes in the first line give the order, so match each to its statement.
2. Soil erosion by running water links with loss of forest cover.

Answer: (c) (i) → (C); (ii) → (B); (iii) → (A); (iv) → (D)

Explanation:
1. Residual fertilizers and pesticides are reduced by using natural fertilizers and drip irrigation: (i) → (C).
2. Animal excreta are converted into manure or biogas: (ii) → (B).
3. Hot water is cooled and recycled before it is discharged: (iii) → (A).
4. Poisonous metallic salts are removed by chemical treatment: (iv) → (D).
5. This is option (c).

Teacher's Note:
1. The book used lower-case letters in the options, but they mean the same column B entries.
2. Hot water and cooling is the easiest pair.

Answer: (d) All of the above

Explanation:
1. In regions of scanty rainfall, water is conserved by drip irrigation, by spraying a chemical film on reservoirs and by growing plants under plastic covers.
2. The book counts all three as methods for the purpose of this question.
3. The answer is (d) All of the above.

Teacher's Note:
1. When every statement is a water-saving measure, 'all of the above' is the answer.
2. Strictly these are water conservation methods in dry regions.

Answer: (d) Alum

Explanation:
1. Alum makes the tiny suspended particles stick together into clumps (flocs) that settle down.
2. Lime, permutit and washing soda are used for softening, not for settling suspended impurities.
3. The answer is alum.

Teacher's Note:
1. Alum is used at water-treatment plants for clarifying water.
2. The process is called coagulation.

Answer: (a) CEADB

Explanation:
1. Washing soda removes both types of hardness by double decomposition: (C).
2. Clark's method converts soluble bicarbonates to insoluble carbonates using lime: (E).
3. Boiling decomposes bicarbonates to insoluble carbonates: (A).
4. The ion exchange method replaces cations and anions with H⁺ and OH⁻: (D).
5. The permutit process replaces Ca²⁺ and Mg²⁺ with Na⁺: (B).
6. The order is C, E, A, D, B: option (a).

Teacher's Note:
1. Permutit gives only Na⁺ in exchange, while a resin uses H⁺ and OH⁻.
2. Boiling decomposes the bicarbonate: Ca(HCO₃)₂ gives CaCO₃ + H₂O + CO₂.

Answer: (c) Decomposition

Explanation:
1. Electrolysis splits water into hydrogen and oxygen: \(\ce{2H2O -> 2H2 + O2}\).
2. One compound breaking down into simpler substances is a decomposition reaction.

Teacher's Note:
1. One reactant and two products means decomposition.
2. Electric current is used to break the compound.

Answer: (a) Demineralized water

Explanation:
1. Laboratory work needs water free from all dissolved ions.
2. Demineralized water is free from mineral ions, whereas potable, desalinated and rain water still contain some ions or impurities.
3. The answer is demineralized water.

Teacher's Note:
1. Ions in water can interfere with reactions and tests.
2. Distilled water is also used for the same purpose.

Answer: (d) Addition of washing soda

Explanation:
1. In every reaction sodium carbonate (washing soda) reacts with the calcium or magnesium salt to give an insoluble carbonate.
2. The bicarbonate removes temporary hardness and the sulphate and chloride remove permanent hardness.
3. The method is addition of washing soda.

Teacher's Note:
1. Look for the common reagent: Na₂CO₃.
2. Clark's method uses lime, not sodium carbonate.

Answer: (b) Fe and Cu

Explanation:
1. Mercury, cadmium, chromium, zinc and arsenic are toxic metals commonly found in industrial effluents.
2. Iron and copper are named by the book as the pair that is not listed among them.
3. The answer is Fe and Cu.

Teacher's Note:
1. Pick the pair with the less poisonous metals.
2. Mercury and cadmium are well-known poisonous pollutants.

Answer: (a) Gases and minerals

Explanation:
1. Pure water is tasteless. Dissolved gases and minerals give drinking water its characteristic taste.
2. The answer is gases and minerals.

Teacher's Note:
1. Water without minerals tastes flat.
2. Sugar is not present in drinking water.

Answer: (d) 3%

Explanation:
1. About 97% of the earth's water is saline (sea) water.
2. Only about 3% is fresh water.
3. The answer is 3%.

Teacher's Note:
1. Saline water and fresh water add up to 100%.
2. Most fresh water is locked in ice caps and glaciers.

Answer: (c) 4°C

Explanation:
1. Water contracts on cooling until 4°C and then expands, so it is densest at 4°C.
2. The answer is 4°C.

Teacher's Note:
1. This is the anomalous expansion of water.
2. Ice floats because it is less dense than water at 4°C.

Answer: (b) specific heat

Explanation:
1. The heat needed to raise the temperature of a unit mass by 1°C is the specific heat.
2. Latent heat is for a change of state at constant temperature.
3. The answer is specific heat.

Teacher's Note:
1. The word 'raise the temperature' points to specific heat.
2. Latent heat does not change the temperature.

Answer: (c) Pure water is a poor conductor of electricity.

Explanation:
1. Water freezes at 0°C only at standard (normal) pressure, so (a) is wrong.
2. The latent heat of fusion of ice is 80 cal/g, and 540 cal/g is the latent heat of vaporization, so (b) is wrong.
3. Water is an excellent solvent, so (d) is wrong.
4. Pure water has no ions, so it is a poor conductor: (c).

Teacher's Note:
1. Check each statement with a known value.
2. Ice: 80 cal/g; steam: 540 cal/g.

Answer: (b) BDACE

Explanation:
1. First the method is named: (B) reverse osmosis.
2. Sea water and pure water are placed in the inner and outer vessels: (D).
3. The two vessels are separated by a semi-permeable membrane: (A).
4. A pressure greater than the osmotic pressure is applied on the sea water: (C).
5. Pure water then passes through the membrane, leaving the salts behind: (E).
6. The order is B, D, A, C, E: option (b).

Teacher's Note:
1. Name the method first, set up the apparatus next, apply pressure last.
2. In reverse osmosis the pressure forces water out of the salty side.

Answer: (c) calcium chloride

Explanation:
1. Hardness is caused by the bicarbonates, chlorides and sulphates of calcium and magnesium.
2. Of the options, only calcium chloride does so: (c).

Teacher's Note:
1. Look for Ca or Mg in the compound.
2. Sodium salts do not cause hardness.

Answer: (b) Extraction of salt from sea water.

Explanation:
1. Rain water harvesting, drip irrigation and spraying cetyl alcohol all help to save and store water.
2. Extracting salt from sea water is an industry that does not conserve or manage the water supply.
3. The answer is (b).

Teacher's Note:
1. Water management means saving, storing or using water wisely.
2. Salt extraction is not about that.

Answer: (d) (i) → (C); (ii) → (A); (iii) → (D); (iv) → (B)

Explanation:
1. Boiling removes temporary hardness: (i) → (C).
2. Clark's method uses calcium hydroxide (slaked lime): (ii) → (A).
3. Permutit is hydrated sodium aluminium silicate: (iii) → (D).
4. Resins give demineralized water: (iv) → (B).
5. This is option (d).

Teacher's Note:
1. Clark's method = lime water.
2. Sodium carbonate (E) is washing soda and is not paired with any entry.

Answer: (a) CBAD

Explanation:
1. Heavy particles settle first in sedimentation: (C).
2. Chemicals such as alum are then added to remove the finer particles: (B).
3. The water is filtered through sand and gravel: (A).
4. Finally microorganisms are removed with chlorine: (D).
5. The order is C, B, A, D: option (a).

Teacher's Note:
1. Killing germs always comes last.
2. Filtration comes after the particles have been made to settle.

Answer: (a) Clark's method

Explanation:
1. Sodium carbonate, the permutit process and ion exchange can remove both types of hardness.
2. Clark's method uses slaked lime, which can only remove temporary hardness.
3. The answer is Clark's method.

Teacher's Note:
1. Lime reacts with bicarbonates, not with chlorides or sulphates.
2. Clark's method = temporary hardness only.

Answer: (b) Distilled water

Explanation:
1. Hard water, sea water and potable water contain dissolved salts.
2. Distilled water is free from salts and impurities, so it is suitable for laboratory work.
3. The answer is distilled water.

Teacher's Note:
1. Hard water would give precipitates with soap and some chemicals.
2. Distillation removes dissolved solids.

Answer: (c) Rain water

Explanation:
1. Sea, pond and underground water all contain dissolved salts or impurities from the earth.
2. Rain water comes straight from the atmosphere as condensed vapour and is the purest natural water.
3. The answer is rain water.

Teacher's Note:
1. Rain water is formed by evaporation and condensation, a natural distillation.
2. It picks up little from the ground.

Answer: (b) 0°C

Explanation:
1. 760 mm of Hg is standard atmospheric pressure.
2. At this pressure pure water freezes at 0°C.
3. The answer is 0°C.

Teacher's Note:
1. Standard pressure is 760 mm Hg or 1 atm.
2. 4°C is the temperature of maximum density, not freezing.

Answer: (a) saline water

Explanation:
1. About 97% of the earth's water is in the seas and oceans, which is saline water.
2. The answer is saline water.

Teacher's Note:
1. Oceans cover most of the earth.
2. Fresh water is only about 3%.