How is safe drinking water produced, and how does it differ from pure water?
Potable water and pure water; treating fresh water; desalination of sea water; and waste water treatment.
A focused answer to AQA GCSE Chemistry 4.10.1, covering the difference between potable and pure water, how fresh water is treated, the desalination of sea water by distillation and reverse osmosis, and how waste water is treated.
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What this dot point is asking
AQA wants you to distinguish potable water from pure water, describe how fresh water is treated to make it potable, describe the desalination of sea water, and outline how waste water is treated. This includes a required practical on analysing and purifying water samples (for example distilling a salt solution).
Potable versus pure water
Treating fresh water
The choice of fresh water as a source is preferred where it is available because it needs only filtering and sterilising, which use far less energy than removing dissolved salts.
Desalination of sea water
Where fresh water is scarce, sea water can be treated to remove the dissolved salts (desalination):
- Distillation: the water is boiled and the steam condensed, leaving the dissolved salts behind.
- Reverse osmosis: the water is forced under pressure through a partially permeable membrane that holds back the dissolved salts.
Both methods need large amounts of energy, so they are expensive and are used mainly where there is no cheaper source of fresh water.
Treating waste water
Waste (sewage) water from homes and industry is treated to remove harmful substances before being returned to rivers or the sea:
- Screening removes large solids and grit.
- Settling lets solids settle out to form sludge, leaving liquid effluent on top.
- Biological treatment: the sludge is broken down by anaerobic bacteria (which also produce useful biogas), and the effluent is treated by aerobic bacteria to break down organic matter and harmful microbes before release.
Try this
Q1. Explain the difference between potable water and pure water. [2 marks]
- Cue. Potable water is safe to drink but contains dissolved substances; pure water contains only water molecules.
Q2. Name two methods used to desalinate sea water. [2 marks]
- Cue. Distillation and reverse osmosis.
Q3. Name a substance used to sterilise water to make it potable. [1 mark]
- Cue. Chlorine (or ozone, or ultraviolet light).
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksDescribe how fresh water from a reservoir is treated to make it potable, and explain why potable water is not the same as pure water in the chemical sense.Show worked answer →
A 4-mark Paper 2 question on water treatment and the meaning of potable.
Treatment (2 marks): pass the water through filter beds to remove insoluble solids and particles; then sterilise it to kill microbes (bacteria and other pathogens) using chlorine, ozone or ultraviolet light. Potable versus pure (2 marks): potable water is safe to drink but still contains low, harmless levels of dissolved salts and other substances, so it is not chemically pure; pure water contains only water molecules.
Markers reward the filter-then-sterilise sequence with a named sterilising agent, and the dissolved-substances point for why potable is not pure.
AQA 20213 marksIn some hot, dry countries, sea water is treated to make it potable. Name two methods used to desalinate sea water, explain how one of them works, and state one disadvantage of desalination.Show worked answer →
A 3-mark question on desalination.
Two methods (1 mark): distillation and reverse osmosis. How one works (1 mark): in distillation the sea water is boiled and the steam (which leaves the dissolved salts behind) is condensed back to pure water; or in reverse osmosis the water is forced through a partially permeable membrane that holds back the dissolved salts. Disadvantage (1 mark): both methods need large amounts of energy, so they are expensive.
Markers reward two named methods, a correct mechanism for one, and the high-energy disadvantage.
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)