methods of separating + purifying substances ×× drinking water

Question 1

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

Answer 1

It does not contain any dissolved ions that might interfere with a chemical analysis.

Question 2

Describe how fresh water can be produced from seawater.

Answer 2

Sea water is heated until it boils. The salt remains in the liquid, and the steam is pure water. The steam is cooled and condensed to make potable water.

Question 3

Describe the steps needed to make fresh water suitable for drinking.

Answer 3

1. large objects are removed by screening using grids
2. a coarse filter bed made from clean sand and gravel removes larger insoluble grit particles
3. aluminium sulfate is added to clump smaller insoluble particles together, which then settle to the bottom in a sedimentation tank
4. a fine filter bed removes very small insoluble particles
5. chlorine gas is added to kill harmful microorganisms

Question 4

Suggest how to purify water when you know what it contains

Answer 4

1. Filtration
2. Sedimentation - a chemical is added which causes tiny solid particles (which would pass through a filter) to clump together into larger particles. These can then be allowed to settle out or may be filtered.
3. Chlorination - chlorine gas is injected into the water to sterilise it. The chlorine kills microbes.

Question 5

Evaluate the hazards and control the risks present when purifying water.

Answer 5

1. Acidic or alkaline water samples - irritating to eyes or skin - rinse off skin if contact occurs, wear eye protection
2. Hot apparatus - Burns - Allow hot apparatus time to cool before touching