Potable Water
Revise Potable Water for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. The required practical analyses and purifies water; potable water is safe to drink, produced by filtering and then sterilising fresh water.
Free downloads
These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Potable water is water that is safe to drink. It is not the same as pure water, because potable water contains dissolved substances — it simply has low enough levels of dissolved salts and microbes to be safe. This is a required practical.
In the UK, rainwater collects in the ground and in lakes and rivers as fresh water. Producing potable water from it involves choosing an appropriate source, passing it through filter beds to remove solids, and sterilising it to kill microbes.
Sterilising agents include chlorine, ozone and ultraviolet light. Where fresh water is scarce, salty water can be desalinated by distillation or by reverse osmosis, but both require large amounts of energy and are therefore expensive.
Revision notes
Potable versus pure
Potable water is safe to drink but is not pure in the chemical sense.
It contains dissolved substances, but the levels of dissolved salts and microbes are low enough to be safe. Pure water contains only H₂O.
Treating fresh water
Choose an appropriate source of fresh water, such as a reservoir.
Pass it through filter beds to remove insoluble solids. Sterilise it to kill microbes, using chlorine, ozone or ultraviolet light.
Desalination
Where fresh water is scarce, salty water can be treated by desalination.
This uses distillation or reverse osmosis through membranes. Both require large amounts of energy, which makes them expensive and limits their use.
Key points
- Potable water is safe to drink.
- Potable water is not chemically pure.
- Filter beds remove insoluble solids.
- Sterilising kills microbes.
- Chlorine, ozone or UV light are used to sterilise.
- Desalination requires large amounts of energy.
Worked examples
Example 1
Explain the difference between potable water and pure water. [2 marks]
Model answer
Potable water is safe to drink but contains dissolved substances at low enough levels to be safe (1 mark). Pure water contains only water molecules and no dissolved substances at all (1 mark).
Example 2
Describe the two main stages in treating fresh water to make it potable. [2 marks]
Model answer
Pass the water through filter beds to remove insoluble solids (1 mark). Sterilise it using chlorine, ozone or ultraviolet light to kill microbes (1 mark).
Example 3
Explain why desalination is not used widely in the UK. [2 marks]
Model answer
Desalination requires large amounts of energy, whether by distillation or reverse osmosis (1 mark), making it expensive, and the UK has adequate supplies of fresh water available more cheaply (1 mark).
Common mistakes
Saying potable water is pure.
It contains dissolved substances; it is simply safe to drink.
Forgetting the sterilising stage.
Filtering alone does not remove microbes.
Naming only one sterilising agent.
Chlorine, ozone and ultraviolet light are all required.
Saying desalination is cheap.
It requires large amounts of energy and is expensive.
Exam tips
- Distinguish potable from pure clearly.
- Give both filtering and sterilising stages.
- Learn all three sterilising agents.
- Explain desalination's limitation through energy cost.
Key terms
- Potable water
- Water that is safe to drink.
- Sterilise
- To kill microbes in water.
- Desalination
- Removing salt from sea water.
- Reverse osmosis
- A desalination method using membranes.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.