Water and Water Treatment Exam Questions

• Sedimentation: large insoluble particles settle to the bottom [1]
• Filtration: water passes through sand/gravel beds to remove smaller insoluble particles [1]
• Chlorination: chlorine is added to kill bacteria/microorganisms [1]
• Additional treatment (fluoridation or pH adjustment) may be applied depending on local requirements [1]

Examiner note: The order matters. Sedimentation comes before filtration. Candidates who write “filter to remove bacteria” lose marks — bacteria are too small to be removed by sand filtration; that is the purpose of chlorination.

(a) Distillation / simple distillation [1]

(b) The sea water is heated until it boils [1]. The water vapour rises into the condenser, cools, and condenses back into liquid water, which is collected — the dissolved salts remain behind in the flask [1]

Examiner note: Filtration cannot separate dissolved salt from water. This is a key distinction. Distillation works because water has a lower boiling point than the dissolved salts. Some candidates confuse distillation with evaporation — evaporation recovers the salt, distillation recovers the water.

• Chlorine kills bacteria / pathogens / microorganisms in the water [1]
• This makes the water safe to drink / prevents waterborne diseases [1]
• Chlorine remains in the water to continue killing bacteria as it travels through pipes to homes [1]

Examiner note: “Chlorine cleans the water” is too vague. Be specific: it kills microorganisms. The residual chlorine point is a strong supplement-level answer that earns extra credit when available.

(a) Sample A is pure water [1] because pure water has a boiling point of exactly 100 degrees C at standard atmospheric pressure, and any dissolved substance raises the boiling point [1]

(b) 0 degrees C [1]

Examiner note: A sharp, fixed boiling point is a test for purity. Sample B contains dissolved impurities, which raise the boiling point above 100 degrees C and lower the freezing point below 0 degrees C.

(a) Desalination is the removal of dissolved salts from sea water to produce fresh/potable water [1]

(b) Distillation [1] and reverse osmosis [1]

(c) Any one from: high energy cost / expensive to run / produces large volumes of concentrated brine waste / environmental impact of brine disposal [1]

Examiner note: Reverse osmosis uses a semi-permeable membrane under high pressure to separate water molecules from dissolved ions. It is more energy-efficient than distillation but still requires significant energy input.

Any three from:

• Insoluble solids / debris — can block pipes and make water unpleasant to drink [1]
• Bacteria / microorganisms — cause waterborne diseases such as cholera and typhoid [1]
• Dissolved toxic substances / heavy metal ions — poisonous and can accumulate in the body [1]
• Excess dissolved salts — can make water taste unpleasant and cause health problems at high concentrations [1]

Examiner note: Give a different reason for each substance. Saying “they are harmful” for all three does not earn full marks. Link each impurity to a specific problem.

• Screening: removes large objects / debris (sticks, leaves, litter) [1]
• Sedimentation: allows smaller insoluble particles / suspended solids to settle to the bottom of a tank [1]
• Filtration: water passes through beds of sand and gravel to remove fine/remaining insoluble particles [1]
• Chlorination: kills bacteria and other microorganisms to make the water safe to drink [1]

Examiner note: Each stage removes a specific category of impurity. The stages become progressively finer in what they remove. This is a common 4-mark structured question — one mark per stage, so be concise but precise.

(a) Heat the water sample in a beaker/flask [1]. Place a thermometer in the water and record the temperature at which the water boils [1]. Observe whether the temperature remains constant during boiling [1]

(b) Pure water boils at a constant/fixed temperature of 100 degrees C (at standard atmospheric pressure) [1]

Examiner note: If the boiling point is above or below 100 degrees C, or if the temperature changes during boiling, the water contains dissolved impurities. An alternative test is to check the freezing point (should be 0 degrees C). Candidates who suggest testing with cobalt chloride paper only show that water is present — this does not prove purity.