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Potable water is water that is safe to drink, with sufficiently low levels of dissolved salts and microbes. Free notes with key terms and 11 questions answered.
What is Potable Water and How Do We Make It? is part of Using the Earth's resources and obtaining potable water in GCSE AQA Higher Chemistry.
4 min
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27
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GCSE AQA Higher
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Potable water is water that is safe to drink, with sufficiently low levels of dissolved salts and microbes. Free notes with key terms and 11 questions answered.
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Use this lesson to stay inside Using the Earth's resources and obtaining potable water while connecting the detail back to the wider Topic 10: Using resources topic.
Read the core explanations from What is Potable Water and How Do We Make It? before testing yourself from memory.
You turn on a tap and clean water flows out. But that water started as rain, picked up dirt and microbes, and somehow became safe to drink. How does that transformation happen?
Here's the crucial distinction many students miss: potable water is NOT pure water. Pure water contains only H₂O molecules, while potable water can contain dissolved substances such as mineral ions. For AQA, the key point is that those dissolved salts and any microbes must be at sufficiently low levels for the water to be safe to drink.
In the UK, we're lucky. Rain provides us with fresh water - water with naturally low levels of dissolved substances. This fresh water collects in rivers, lakes and underground aquifers.
But here's the problem: fresh water isn't automatically safe to drink. It picks up harmful microbes from soil, animal waste, and pollution. It also contains particles of dirt and organic matter that make it cloudy and unpleasant.
The methods used to make water potable depend entirely on the local water source and conditions. Coastal areas might use seawater desalination, while areas with good rainfall use river water treatment.
In the UK, we turn fresh water into potable water using three essential steps. Each step tackles a different problem with the raw water.
Worked example: UK Water Treatment Process
How do we convert fresh river water into safe drinking water?
Answer: Clean, safe potable water ready for distribution
Notice the logical sequence: you can't sterilise dirty water effectively, so filtration comes first. But filtration alone won't kill microbes, so sterilisation is essential.
Sterilisation is your last line of defence against harmful microbes. Three main methods are used, each with different advantages.
Chlorine is the most common choice. It's cheap, effective, and provides ongoing protection as water travels through pipes. However, some people dislike the taste and smell.
Ozone is very effective and leaves no taste, but it's expensive and provides no protection once the water leaves the treatment plant.
Ultraviolet light kills microbes without adding chemicals, but again provides no ongoing protection and requires clear water to work effectively.
Each step in water treatment targets specific contaminants. Understanding why each step is necessary helps you explain the complete process.
Source selection is crucial because starting with seawater would require expensive desalination. Fresh water already has low dissolved salts, making treatment much simpler and cheaper.
Filtration removes physical particles that would interfere with sterilisation. Cloudy water blocks UV light and provides hiding places for microbes where chemicals can't reach them.
Sterilisation is the final step because it's most effective on clear, filtered water. It targets the invisible threat - harmful microbes that cause diseases like cholera and typhoid.
Water treatment questions often ask you to explain the reasons for each step. Examiners want to see that you understand the science, not just memorised the process.
Common exam mistakes include confusing potable water with pure water, forgetting to explain WHY filtration comes before sterilisation, and not linking the choice of method to local water sources and conditions.
Exam precision note: In What is Potable Water and How Do We Make It? - Potable water - Using the Earth's resources and obtaining potable water, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
Local conditions decide which method is used to obtain potable water. Areas with high rainfall can often treat fresh water from rivers, lakes, and aquifers. Drier regions may need desalination of seawater, but this is much more energy-intensive because the dissolved salts must be removed rather than simply filtered out. That is why the AQA specification emphasises source choice before treatment steps.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-25.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Potable water is water that is safe to drink, with sufficiently low levels of dissolved salts and microbes. It is not chemically pure, because it can still contain dissolved substances such as mineral ions.
Fresh water is water with low levels of dissolved substances, typically from rain that collects in rivers, lakes and underground aquifers. It still isn't automatically safe to drink, because it picks up microbes and dirt.
In the UK, potable water is produced by choosing a fresh water source such as a river, lake or groundwater, passing it through filter beds of sand and gravel to remove solids, then sterilising it with chlorine, ozone or ultraviolet light to kill microbes.
Pure water contains only H₂O molecules, while potable water can contain dissolved substances such as mineral ions, as long as the dissolved salts and microbes are at low enough levels for it to be safe to drink.
Drinking water is sterilised with chlorine, ozone or ultraviolet light. Chlorine is cheap and gives ongoing protection in pipes, though some dislike the taste; ozone leaves no taste but is expensive; ultraviolet light adds no chemicals; neither ozone nor UV protects water once it leaves the plant.
Filtration comes before sterilisation because dirty water cannot be sterilised effectively: cloudy water blocks UV light and gives microbes hiding places where chemicals cannot reach them. Filtration alone does not kill microbes, so sterilisation is still essential.
Chlorine is the most common sterilising agent for drinking water because it is cheap, effective and gives ongoing protection as water travels through pipes. However, some people dislike its taste and smell.
Ozone is very effective and leaves no taste, but it is expensive, and UV light kills microbes without adding chemicals but needs clear water. Neither gives any protection once the water leaves the treatment plant, unlike chlorine.
Seawater isn't usually used for drinking water in the UK because it would need expensive desalination to remove the dissolved salts. Fresh water from rain already has low dissolved salts, so treatment is simpler and cheaper.
Harmful microbes in raw water can cause diseases like cholera and typhoid. Sterilisation with chlorine, ozone or UV light is the final step of water treatment because it kills these microbes.
Filter beds of sand and gravel remove solid particles, dirt and some larger microbes from water. Filtration comes before sterilisation because cloudy water blocks UV light and gives microbes hiding places where chemicals can't reach them.