Drinking water

Question 1

What must potable water have?

Answer 1

Low levels of contaminating substances.
Low levels of microbes.

Question 2

What are the stages in water treatment?

Answer 2

1) Sedimentation - large insoluble particles sink to the bottom of a tank.
2) Filtration - small insoluble particles are removed by filtering through beds of sand.
3) Chlorination - chlorine gas is bubbled from the water to kill microbes.

Question 3

How is distillation used on sea water?

Answer 3

Sea water contains dissolved salts. Their concentration is too high to drink it safely. Sea water can be made potable by simple distillation: filtered sea water is boiled, and then the water vapour is cooled and condensed to form distilled water.

Question 4

Evaluate simple distillation of sea water.

Answer 4

Advantages - uses a plentiful raw material, produces pure water, and kills microbes in the sea water.

Disadvantage - needs a lot of energy to heat the water.

Distillation is rarely used in the UK, which has high fuel costs. It is suitable for countries with low fuel costs, little fresh water, or plentiful sunshine to evaporate the water without using fuel.

Question 5

Explain why water used for chemical analysis must not contain any dissolved salts.

Answer 5

Dissolved salts could react with the substances used in the analysis. A product formed in the reaction could interfere with the analysis, giving a false result. If the water used does not contain any dissolved salts, this will not happen.