Potable water: If supplies of fresh water are limited, desalination of salty water or sea water may be required. Desalination can be done by distillation or by processes that use membranes such as reverse osmosis. These processes require large amounts of energy.

The idea

Fresh water is scarce in some regions, so salty water or sea water must be treated to make it potable. Desalination removes dissolved salts. Distillation heats the salty water until it boils, condenses the steam and collects the distillate, leaving salts behind. Reverse osmosis forces salty water at high pressure through a partially permeable membrane that retains dissolved ions while water molecules pass through. Both methods demand large amounts of energy: distillation mainly for heating and cooling, reverse osmosis mainly for pumping water at high pressure. Energy use makes desalination expensive and can release carbon dioxide if the electricity comes from burning fossil fuels.

For example: A coastal town with little rainfall may rely on a reverse osmosis plant, whereas a small-scale distillation unit can be used where a reliable heat source exists.

What earns the marks

• Desalination is needed when fresh water supplies are limited and salty water or sea water must be made potable.
• Distillation involves boiling salty water, condensing the water vapour and collecting the liquid distillate, which leaves dissolved salts behind.
• Reverse osmosis uses a partially permeable membrane and high pressure so water passes through while dissolved ions are retained.
• Comparing methods: distillation uses energy mainly for heating and cooling, while reverse osmosis uses energy mainly for pumping water at high pressure.
• The product of desalination is water with a much lower concentration of dissolved salts, suitable for drinking after appropriate treatment.

Common mistakes and how to put them right

• Thinking that distillation simply filters out salt.

  Put it right by explaining that the water changes to vapour and then condenses, leaving dissolved salts in the original container.

• Believing that reverse osmosis works by boiling water.

  Put it right by stating that it uses pressure and a partially permeable membrane, not a change of state.

• Assuming desalination is cheap because sea water is abundant.

  Put it right by linking both processes to large energy demands and therefore high cost.

Exam tips

• Name the two processes clearly and state the key principle of each: boiling and condensing for distillation, pressure and a partially permeable membrane for reverse osmosis.
• When asked why desalination is not always used, refer to the large energy requirement and the resulting cost or environmental impact.
• Use comparative language such as 'whereas' or 'in contrast' when contrasting distillation with reverse osmosis, and keep the comparison focused on the separation mechanism and energy use.