AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Potable water: describe the differences in treatment of ground water and salty water
Ground water is usually treated by screening to remove large objects, then filtration to remove insoluble particles, followed by sterilisation using chlorine, ozone or ultraviolet light to kill…
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Ground water is usually treated by screening to remove large objects, then filtration to remove insoluble particles, followed by sterilisation using chlorine, ozone or ultraviolet light to kill microorganisms. Salty water, such as sea water, needs desalination because dissolved sodium chloride cannot be removed by filtration or sterilisation. Desalination is carried out by distillation or by reverse osmosis, both of which require significant energy. Distillation involves heating the salty water so that pure water evaporates and is then condensed, leaving dissolved salts behind. Reverse osmosis forces water through a membrane that retains dissolved ions. In both cases the product is potable water, but the processes differ because ground water mainly needs removal of microbes and insoluble solids, while salty water needs removal of dissolved salts.
What earns the marks
- Ground water treatment typically involves screening, filtration to remove insoluble particles, and sterilisation with chlorine, ozone or ultraviolet light.
- Salty water treatment requires desalination because dissolved salts cannot be removed by filtration or sterilisation alone.
- Desalination can be carried out by distillation, in which water is evaporated and condensed, leaving dissolved salts behind.
- Desalination can also be carried out by reverse osmosis, in which water is forced through a membrane that retains dissolved ions.
- The product of both treatment routes is potable water, but the methods differ because ground water mainly contains insoluble particles and microbes, while salty water contains dissolved salts.
Common mistakes and how to put them right
Stating that salty water can be made potable by filtration alone.
Put it right: Filtration removes insoluble particles but not dissolved salts, so desalination by distillation or reverse osmosis is needed.
Confusing sterilisation with desalination.
Put it right: Sterilisation kills microorganisms but does not remove dissolved sodium chloride.
Claiming that distillation removes microbes only.
Put it right: Distillation removes dissolved salts as well as microbes, because the water evaporates and leaves non-volatile substances behind.
Exam tips
- Name the stages in order for ground water: screening, filtration, sterilisation.
- For salty water, name desalination and give either distillation or reverse osmosis as the method.
- Mention the high energy cost of desalination to show why it is used only when fresh water is scarce.
Try the lesson
The start of the Potable water: describe the differences in treatment of ground water and salty water lesson, made from checked teaching notes for this specification point.
Have a go first
Ground water and salty water are both treated to make potable water. Which statement describes a difference in their treatment?
Listen
Ground water is usually treated by screening to remove large objects, then filtration to remove insoluble particles, followed by sterilisation using chlorine, ozone or ultraviolet light to kill microorganisms. Salty water, such as sea water, needs desalination because dissolved sodium chloride cannot be removed by filtration or sterilisation. Desalination is carried out by distillation or by reverse osmosis, both of which require significant energy. Distillation involves heating the salty water so that pure water evaporates and is then condensed, leaving dissolved salts behind. Reverse osmosis forces water through a membrane that retains dissolved ions. In both cases the product is potable water, but the processes differ because ground water mainly needs removal of microbes and insoluble solids, while salty water needs removal of dissolved salts. A strong answer earns marks for these points. Ground water treatment typically involves screening, filtration to remove insoluble particles, and sterilisation with chlorine, ozone or ultraviolet light. Salty water treatment requires desalination because dissolved salts cannot be removed by filtration or sterilisation alone. Desalination can be carried out by distillation, in which water is evaporated and condensed, leaving dissolved salts behind. Desalination can also be carried out by reverse osmosis, in which water is forced through a membrane that retains dissolved ions. A common slip: stating that salty water can be made potable by filtration alone. Instead, filtration removes insoluble particles but not dissolved salts, so desalination by distillation or reverse osmosis is needed. Exam tip: Name the stages in order for ground water: screening, filtration, sterilisation.
See it
Key words
Say what each word means before you turn the card.
Desalination
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Distillation
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Reverse osmosis
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See every definition
- Desalination
- Treatment needed for salty water because dissolved sodium chloride cannot be removed by filtration or sterilisation, carried out by distillation or reverse osmosis.
- Distillation
- Heating the salty water so that pure water evaporates and is then condensed, leaving dissolved salts behind.
- Reverse osmosis
- Forcing water through a membrane that retains dissolved ions.
Label the diagram
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