AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Desalination can be done by distillation or by processes that use…
Full specification wording
Potable water: Desalination can be done by distillation or by processes that use membranes such as reverse osmosis.
Desalination removes dissolved salts from salty water or sea water.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 9 of 12
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Desalination removes dissolved salts from salty water or sea water. One method is distillation: the water is heated to boiling, the vapour is cooled and condensed, leaving dissolved salts behind in the original container. Another method uses membranes, such as reverse osmosis. In reverse osmosis, salty water is forced at high pressure through a partially permeable membrane that allows water molecules through but retains most dissolved ions. Both methods produce water with low levels of dissolved salts, but both require significant energy. Distillation uses energy for heating and cooling, while reverse osmosis uses energy to generate high pressure. Understanding both methods means recognising that distillation relies on a change of state and that membrane processes rely on selective passage of water molecules.
What earns the marks
- Desalination can be carried out by distillation.
- Distillation involves boiling salty water and condensing the vapour, leaving dissolved salts behind.
- Desalination can be carried out using membranes such as reverse osmosis.
- Reverse osmosis forces water through a partially permeable membrane under pressure, retaining dissolved ions.
Common mistakes and how to put them right
Saying distillation removes salts by filtering.
Put it right: Distillation separates water by boiling and condensing, leaving dissolved salts behind.
Describing reverse osmosis as simple filtration.
Put it right: Reverse osmosis uses pressure and a partially permeable membrane to retain dissolved ions.
Claiming membrane processes remove all particles including water molecules.
Put it right: Water molecules pass through while most dissolved ions are retained.
Exam tips
- For distillation, state the sequence: heat to boil, condense the vapour, collect the distillate.
- For reverse osmosis, mention high pressure and a partially permeable membrane.
- Compare the two methods by noting that both remove dissolved salts but use energy in different ways.
Try the lesson
The start of the Potable water: Desalination can be done by distillation or by processes that use membranes such as reverse osmosis. lesson, made from checked teaching notes for this specification point.
Have a go first
Which two methods can be used to desalinate salty water?
Listen
Desalination removes dissolved salts from salty water or sea water. One method is distillation: the water is heated to boiling, the vapour is cooled and condensed, leaving dissolved salts behind in the original container. Another method uses membranes, such as reverse osmosis. In reverse osmosis, salty water is forced at high pressure through a partially permeable membrane that allows water molecules through but retains most dissolved ions. Both methods produce water with low levels of dissolved salts, but both require significant energy. Distillation uses energy for heating and cooling, while reverse osmosis uses energy to generate high pressure. Understanding both methods means recognising that distillation relies on a change of state and that membrane processes rely on selective passage of water molecules. A strong answer earns marks for these points. Desalination can be carried out by distillation. Distillation involves boiling salty water and condensing the vapour, leaving dissolved salts behind. Desalination can be carried out using membranes such as reverse osmosis. Reverse osmosis forces water through a partially permeable membrane under pressure, retaining dissolved ions. A common slip: Saying distillation removes salts by filtering. Instead, distillation separates water by boiling and condensing, leaving dissolved salts behind. Exam tip: For distillation, state the sequence: heat to boil, condense the vapour, collect the distillate.
Key words
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Separation by distillation
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Membrane separation and reverse osmosis
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Dissolved salts and water purity
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Energy demands of desalination
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See every definition
- Separation by distillation
- The water is heated to boiling, the vapour is cooled and condensed, leaving dissolved salts behind in the original container.
- Membrane separation and reverse osmosis
- Salty water is forced at high pressure through a partially permeable membrane that allows water molecules through but retains most dissolved ions.
- Dissolved salts and water purity
- Both methods produce water with low levels of dissolved salts.
- Energy demands of desalination
- Both methods require significant energy, which affects their cost and practicality.
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