Potable water: give reasons for the steps used to produce potable water.

The idea

Potable water is water that is safe to drink. It is not pure water in the chemical sense because it contains dissolved substances, but the levels of microbes and contaminants are low enough not to harm health. The steps used depend on the source. For most UK sources, water is passed through filter beds of sand and gravel to remove insoluble particles such as grit and plant material. Chlorine is then added to kill microorganisms. If the water is very salty, for example sea water, simple filtration and chlorination are not enough because dissolved salts remain. Desalination by distillation or reverse osmosis removes dissolved salts, but it needs large amounts of energy, so it is expensive.

What earns the marks

• Filtration through filter beds removes insoluble solids such as grit, soil and plant material; it does not remove dissolved salts or microorganisms.
• Chlorination kills microorganisms such as bacteria that could cause disease; it does not remove dissolved solids.
• For water containing high levels of dissolved salts, desalination by distillation or reverse osmosis is needed because filtration and chlorination cannot remove dissolved salts.
• Distillation involves boiling water to produce steam and condensing the steam, leaving dissolved salts behind; reverse osmosis uses pressure to force water through a membrane that retains dissolved ions.
• The choice of steps depends on the source of the water and the impurities it contains, so the reasons must be linked to the specific impurity being removed.
• Potable water is safe to drink but is not pure water because it still contains some dissolved substances at acceptable levels.

Common mistakes and how to put them right

• Stating that filtration removes dissolved salts.

  Put it right: Filtration removes insoluble solids only; dissolved salts require desalination.

• Saying that chlorine removes dirt or makes water pure.

  Put it right: Chlorine kills microorganisms; it does not remove insoluble or dissolved solids.

• Describing distillation as freezing or as simply boiling.

  Put it right: Distillation involves boiling to produce steam and then condensing the steam to produce liquid water.

• Claiming that potable water is pure water.

  Put it right: Potable water contains dissolved substances but at levels safe for drinking.

Exam tips

• Link each step to the impurity it removes: filtration for insoluble solids, chlorination for microorganisms, desalination for dissolved salts.
• Use the phrase 'safe to drink' rather than 'pure' when describing potable water.
• If asked to compare methods, state that desalination is more expensive because it uses more energy than filtration and chlorination.
• Read the question carefully: if it asks for reasons, give a because clause for each step rather than just naming the step.