Potable water — AQA GCSE Chemistry
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Potable water explained
Potable water covers 12 specification points, including Water of appropriate quality is essential for… and For humans, drinking water should have….
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Open a point below for its explanation and revision guidance.
Water of appropriate quality is essential for life.
Water of appropriate quality means water that is safe for its intended use. For drinking, this is potable water: water with sufficiently low levels of dissolved salts and microbes to be safe to drink. Potable water is not pure water in the chemical sense, because it still contains dissolved substances. Different uses need different quality: drinking requires potable water, whereas washing, irrigation and industrial cooling may use lower-quality water. Water for life also includes water for agriculture and sanitation, so appropriate quality depends on the purpose. In the UK, potable water is produced by choosing a suitable source, such as a river, lake or aquifer, passing it through filter beds to remove solids, then sterilising it to kill microbes, for example with chlorine, ozone or ultraviolet light.
For humans, drinking water should have sufficiently low levels of dissolved salts and microbes.
Drinking water must be safe for humans, which means keeping dissolved salts and microbes at sufficiently low levels. High dissolved salts can affect taste and, in extreme cases, health, while microbes such as bacteria can cause disease. Treatment therefore aims to reduce both. For example, water from a river may be filtered to remove solids, then chlorinated to kill microbes; in coastal areas, desalination may reduce salt levels. The phrase 'sufficiently low' shows that water need not be chemically pure, but must meet safety standards. Students should link the type of impurity to the treatment method and explain why each is reduced.
Water that is safe to drink is called potable water.
Potable water is water that is safe for humans to drink. It is not the same as chemically pure water, because it can contain low levels of dissolved substances and microbes, provided these are sufficiently low to be safe. The term 'potable' is used to distinguish drinking water from other water, such as sea water or waste water, which may be unsafe. For example, tap water in the UK is potable: it has been treated to remove solids and kill microbes, but it still contains some dissolved ions. Students should use the term correctly and explain that potability is about safety, not purity.
Potable water is not pure water in the chemical sense because it contains dissolved substances.
In chemistry, pure water contains only H₂O molecules. Potable water is not pure in this sense because it contains dissolved substances, such as mineral ions, at low concentrations. These dissolved substances do not necessarily make the water unsafe, but they mean it is a mixture rather than a pure substance. For example, tap water may contain dissolved calcium and magnesium ions, which can cause temporary hardness. Students should distinguish between 'pure' as used in everyday language (meaning clean or safe) and 'pure' as used in chemistry (meaning a single substance). This distinction is important when interpreting tests for purity, such as boiling point.
The methods used to produce potable water depend on available supplies of water and local conditions.
The treatment used to make water potable is not the same everywhere. It depends on what water is available and local conditions. For example, in the UK, rain collected in reservoirs or aquifers is usually treated by filtration to remove solids and chlorination to kill microbes. In dry coastal areas, sea water may be desalinated by distillation or reverse osmosis, which is expensive and energy-intensive. If water contains high levels of dissolved salts, desalination is needed; if it mainly contains microbes, chlorination may be sufficient. Students should be able to choose a suitable treatment based on the source and explain why it is appropriate.
In the United Kingdom (UK), rain provides water with low levels of dissolved substances (fresh water) that collects in the ground and in lakes and rivers, and most potable water is produced by: choosing an appropriate source of fresh water passing the water through filter beds sterilising.
Potable water is water that is safe to drink, meaning it has sufficiently low levels of dissolved salts and microbes. In the UK, rainfall is naturally low in dissolved substances, so it is a useful fresh water source; it collects underground in aquifers and in lakes and rivers. To produce potable water, an appropriate fresh water source is first chosen, considering factors such as local availability, reliability of supply and the level of dissolved substances. The water is then passed through filter beds, which remove insoluble solids such as grit, soil and small particles. Finally it is sterilised to kill microbes. This sequence treats the water without removing all dissolved substances, because potable water is not pure water in the chemical sense.
Sterilising agents used for potable water include chlorine, ozone or ultraviolet light.
After insoluble solids have been removed by filter beds, potable water must be sterilised to kill microbes that could cause disease. The sterilising agents specified are chlorine, ozone and ultraviolet light. Chlorine is added as a gas or in a solution and kills microbes by disrupting their cell processes; it also provides some residual protection as water travels through pipes. Ozone is a reactive gas that destroys microbes by oxidation. Ultraviolet light is used by passing water through a chamber where UV radiation damages the genetic material of microbes so they cannot reproduce. Each method kills microbes but does not remove dissolved substances, so sterilisation is not a purification step in the chemical sense.
If supplies of fresh water are limited, desalination of salty water or sea water may be required.
Some regions have limited fresh water because of low rainfall, high demand or both. In these situations, salty water from the sea or from underground sources may be treated by desalination to produce potable water. Desalination removes dissolved salts so the water becomes safe to drink. It is used when fresh water supplies are insufficient, for example in dry coastal areas or countries with high water demand. Desalination is expensive and uses a lot of energy, so it is not the first choice where fresh water is available. Understanding when desalination is required means linking water scarcity to the need for an alternative source and recognising that salty water must have its dissolved salts removed.
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. 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.
These processes require large amounts of energy.
The processes that make water potable can demand significant energy, especially when water is scarce or salty. In the UK, most potable water comes from ground water or rivers and lakes, and treatment usually involves filtration to remove insoluble particles and chlorination to kill microbes; these steps are relatively low in energy compared with desalination. Desalination of salty water, by distillation or reverse osmosis, requires much more energy because water must be heated to boiling and condensed, or forced at high pressure through a membrane. For example, heating 1 kg of water from 20 °C to 100 °C requires about 3.4 × 10⁵ J, and the energy demand rises further when large volumes are treated. This explains why desalination is used mainly where fresh water is limited, such as in some dry coastal regions, and why reducing energy use is a key design aim.
Students should be able to: distinguish between potable water and pure water describe the differences in treatment of ground water and salty water give reasons for the steps used to produce potable water.
Potable water is water that is safe to drink; it contains dissolved substances but has sufficiently low levels of microbes and contaminants. Pure water, in a chemistry context, contains only H₂O molecules and no dissolved substances. Ground water is usually treated by filtration to remove insoluble particles and sterilisation, often by chlorination, to kill microbes. Salty water needs desalination, for example by distillation or reverse osmosis, because dissolved salts must be removed. Each step has a purpose: filtration removes solids, chlorination kills microorganisms, distillation boils and condenses water leaving salts behind, and reverse osmosis forces water through a membrane that retains ions. Understanding these differences allows students to select and justify treatment methods for a given water source.
Required practical 8: analysis and purification of water samples from different sources, including pH, dissolved solids and distillation.
In this required practical, students analyse water samples from different sources and purify one sample by distillation. Analysis includes measuring pH using universal indicator or a pH meter, and determining dissolved solids by evaporating a measured volume of water and weighing the residue. Distillation involves heating the water sample so that water vaporises, leaving dissolved solids behind, and then condensing the vapour to collect pure water. Students record observations, compare samples and explain how results show differences in water quality. A larger residue after evaporation indicates more dissolved solids. The practical develops skills in accurate measurement, safe heating and separation techniques.
Your focus
- Define potable water and explain why it is not pure water.
- Describe how potable water is produced from a suitable source.
- Explain why different uses require water of different quality.
Show all 36 objectives
- Describe why drinking water must have low levels of dissolved salts and microbes.
- Identify which treatment methods reduce salts and which reduce microbes.
- Explain the meaning of 'sufficiently low' in the context of potable water.
- Define potable water as water that is safe to drink.
- Explain why potable water is not necessarily pure water.
- Use the term 'potable' accurately in context.
- Explain why potable water is not chemically pure.
- Describe the difference between pure water and potable water.
- Identify dissolved substances as the reason potable water is a mixture.
- Describe how treatment methods vary with water source and local conditions.
- Select a suitable method to produce potable water from a given source.
- Explain why desalination is used in some areas but not others.
- State that potable water is safe to drink and contains low levels of dissolved substances and microbes.
- Describe how UK rain provides fresh water that collects in the ground and in lakes and rivers.
- Outline the three stages used to produce most potable water: choosing a source, filtering and sterilising.
- Name chlorine, ozone and ultraviolet light as sterilising agents for potable water.
- Explain that sterilising agents kill microbes rather than removing dissolved substances.
- Recognise that sterilisation is a distinct stage from filtration in water treatment.
- State that desalination may be required when fresh water supplies are limited.
- Identify salty water and sea water as sources that can be desalinated.
- Explain that desalination removes dissolved salts to make water potable.
- Describe distillation as a desalination method involving boiling and condensation.
- Describe reverse osmosis as a membrane process using pressure to remove dissolved salts.
- Compare distillation and reverse osmosis in terms of how they separate water from dissolved salts.
- Describe why some water treatment processes require large amounts of energy.
- Compare the energy demands of treating fresh water and salty water.
- Explain how energy use influences the choice and cost of producing potable water.
- Distinguish between potable water and pure water in terms of composition and safety.
- Describe the different treatment methods used for ground water and salty water.
- Explain the purpose of filtration, chlorination, distillation and reverse osmosis in producing potable water.
- Carry out tests to measure pH and dissolved solids in water samples.
- Purify a water sample by distillation and collect the distillate safely.
- Interpret results to compare water samples and explain how distillation removes dissolved solids.
Potable water exam tips
Marking Points
- Potable water is water that is safe to drink, with sufficiently low levels of dissolved salts and microbes.
- Potable water is not pure water because it contains dissolved substances.
- Different uses require different water quality, for example drinking, washing, irrigation and industrial cooling.
- Water of appropriate quality is essential for life because it supports drinking, sanitation, agriculture and industry.
- In the UK, potable water is produced by selecting a source, filtering to remove solids and sterilising to kill microbes.
- States that drinking water must have low levels of dissolved salts to be safe and palatable.
- States that drinking water must have low levels of microbes to prevent disease.
- Explains that 'sufficiently low' means water need not be chemically pure, only safe for consumption.
- Links specific treatments to the impurity they reduce, such as chlorination for microbes or desalination for salts.
- Recognises that safety standards for potable water are set to protect human health.
- Defines potable water as water that is safe to drink.
- Distinguishes potable water from chemically pure water.
- States that potable water may contain low levels of dissolved substances and microbes.
- Uses the term 'potable' correctly in written answers.
- Recognises that potability depends on safety standards rather than absence of all impurities.
- States that pure water in chemistry contains only H₂O molecules.
- Explains that potable water contains dissolved substances, so it is a mixture.
- Distinguishes chemical purity from potability.
- Gives examples of dissolved substances in potable water, such as mineral ions.
- Links the presence of dissolved substances to properties such as boiling point or hardness.
- States that the choice of treatment depends on the water source and local conditions.
- Describes typical treatments for fresh water, such as filtration and chlorination.
- Describes desalination for sea water or water with high salt levels.
- Explains why desalination is used in some locations but not others, linking to energy cost or availability.
- Selects an appropriate method for a given water source and justifies the choice.
- Potable water is water that is safe to drink, with low levels of dissolved substances and microbes.
- UK rain is naturally low in dissolved substances, so it is classed as fresh water.
- Fresh water collects in the ground, including aquifers, and in lakes and rivers.
- Choosing an appropriate source considers availability, reliability and the level of dissolved substances present.
- Filter beds remove insoluble solids such as grit, soil and particles; they do not remove dissolved substances.
- Sterilising kills microbes and is the final treatment step described for producing potable water.
- Sterilising kills microbes in water so that it is safe to drink.
- Chlorine is a sterilising agent used for potable water.
- Ozone is a sterilising agent used for potable water.
- Ultraviolet light is a sterilising agent used for potable water.
- Sterilisation does not remove dissolved substances; it targets microbes.
- Sterilisation follows filtration in the treatment of potable water.
- Desalination is used when supplies of fresh water are limited.
- Salty water or sea water can be treated by desalination.
- Desalination removes dissolved salts to produce potable water.
- Desalination is an alternative when fresh water sources are insufficient.
- Desalination is energy-intensive and costly compared with using fresh water.
- The need for desalination depends on local availability of fresh water.
- 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.
- Both distillation and reverse osmosis produce water with low levels of dissolved salts.
- Both methods require energy, which affects their cost and practicality.
- States that potable water production can require large amounts of energy, especially when the source is salty water.
- Identifies distillation as an energy-intensive desalination method because it involves boiling water and condensing the vapour.
- Identifies reverse osmosis as a desalination method that requires energy to pressurise water through a membrane.
- Compares the energy demand of treating fresh water (filtration and chlorination) with desalinating salty water.
- Explains that energy use affects the cost and environmental impact of supplying potable water.
- Links the choice of water treatment method to local availability of fresh water and energy resources.
- Defines potable water as water that is safe to drink and pure water as water containing only H₂O molecules.
- States that potable water may contain dissolved substances but must have low levels of microbes and harmful contaminants.
- Describes ground water treatment as filtration followed by sterilisation, such as chlorination.
- Describes salty water treatment as desalination by distillation or reverse osmosis.
- Explains that filtration removes insoluble particles and chlorination kills microorganisms.
- Explains that distillation removes dissolved salts by boiling and condensing water, while reverse osmosis uses pressure and a membrane.
- Gives reasons for each step in terms of the type of impurity being removed.
- Measures the pH of water samples using universal indicator or a pH meter and records the values.
- Evaporates a measured volume of water to dryness and weighs the residue to compare dissolved solids.
- Sets up distillation apparatus correctly, with a heat source, condenser and collection vessel.
- Heats the water sample to boiling, condenses the vapour and collects the distillate.
- Compares the distilled water with the original sample, for example by pH or residue test.
- Explains that distillation removes dissolved solids and produces water with lower levels of impurities.
Examiner Tips
- 💡Define potable water in terms of safety and low levels of dissolved salts and microbes, not purity.
- 💡When describing treatment, name the stages in order and state the purpose of each stage.
- 💡Link water quality to a specific use to show why appropriate quality matters.
- 💡When asked why water is treated, name the impurity and the risk it poses, such as microbes causing disease.
- 💡Use the phrase 'sufficiently low' to show understanding that potable water is not chemically pure.
- 💡If given a water source, match the treatment to the main impurity present, for example chlorination for river water.
- 💡Avoid saying water is 'pure' unless you mean chemically pure; use 'potable' or 'safe to drink' instead.
- 💡Learn the definition word-for-word: water that is safe to drink.
- 💡In explanations, contrast potable water with pure water to show the difference.
- 💡Use the term 'potable' rather than 'drinking water' when the question asks for the scientific term.
- 💡If asked to identify potable water, look for evidence of safety, not just clarity.
- 💡When a question asks about purity, state that pure water contains only H₂O molecules.
- 💡Use the phrase 'in the chemical sense' to show you understand the distinction.
- 💡If asked why potable water is not pure, refer to dissolved substances.
- 💡Avoid using 'pure' to mean 'clean' in chemistry answers.
- 💡When given a water source, identify the main impurity before choosing a treatment.
- 💡Use local context, such as coastal or arid regions, to justify desalination.
- 💡Compare methods by cost and energy use where the question asks for evaluation.
- 💡Remember that potable water production must be safe, so multiple steps may be needed.
- 💡Link each treatment step to what it removes: filter beds remove insoluble solids, sterilising kills microbes.
- 💡Use the phrase low levels of dissolved substances when describing UK rain rather than saying it has no dissolved substances.
- 💡When asked to sequence the process, write the three stages in order: choose source, filter, sterilise.
- 💡Learn all three sterilising agents as a set: chlorine, ozone and ultraviolet light.
- 💡If asked why sterilisation is needed, refer to killing microbes that could cause disease.
- 💡Avoid saying sterilisation makes water pure; it makes water safe by killing microbes.
- 💡Link desalination to the phrase supplies of fresh water are limited in your answer.
- 💡State that desalination removes dissolved salts, not just microbes or insoluble solids.
- 💡Mention that desalination is expensive or energy-intensive when discussing why it is not always used.
- 💡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.
- 💡When asked why desalination is expensive, link your answer to the energy needed for boiling or high-pressure pumping, not just to the equipment.
- 💡Use comparative language such as 'more energy than' when contrasting salty water treatment with fresh water treatment.
- 💡If a question gives data about energy costs, quote the relevant values and state what they show about the process.
- 💡When comparing ground water and salty water, name the specific treatment steps and the impurity each step removes.
- 💡Use the term 'sterilisation' or 'chlorination' for killing microbes, and 'desalination' for removing salts.
- 💡If asked to give reasons, link each step to the problem it solves, such as 'filtration removes solid particles that would make water cloudy or unsafe'.
- 💡When describing the practical, state the volume of water used for evaporation and the method for weighing the residue.
- 💡Draw and label the distillation apparatus clearly, including the direction of water flow in the condenser.
- 💡Link the results to water quality: a higher residue mass indicates more dissolved solids, and a pH away from 7 indicates acidity or alkalinity.
Common Mistakes
- Stating that potable water is pure water; the correction is that potable water contains dissolved substances and is safe rather than chemically pure.
- Confusing sterilising with filtering; the correction is that filtering removes insoluble solids while sterilising kills microbes.
- Assuming all water uses need drinking-quality water; the correction is that appropriate quality depends on the purpose, so irrigation and cooling may use lower-quality water.
- Thinking that drinking water must be 100% pure water with no dissolved substances; correction: potable water can contain low levels of dissolved salts and must simply be safe.
- Confusing the removal of microbes with the removal of salts; correction: chlorination or UV treatment targets microbes, while desalination or distillation targets dissolved salts.
- Assuming all dissolved salts are equally harmful; correction: the requirement is that levels are sufficiently low, not that all salts are absent.
- Defining potable water as pure water; correction: potable water is safe to drink and may contain dissolved substances.
- Using 'potable' to mean 'portable'; correction: potable means safe to drink.
- Assuming all clear water is potable; correction: clear water may still contain harmful microbes or dissolved salts.
- Calling potable water pure because it is clear; correction: chemical purity means only one substance is present.
- Thinking dissolved substances always make water unsafe; correction: low levels can be safe and are allowed in potable water.
- Confusing a mixture with a compound; correction: potable water is a mixture of water and dissolved substances.
- Assuming all water is treated in the same way; correction: treatment depends on the impurities present and local factors.
- Thinking desalination is always used; correction: it is used where fresh water is scarce or salt levels are high.
- Confusing filtration with chlorination; correction: filtration removes insoluble solids, chlorination kills microbes.
- Describing potable water as pure water; potable water still contains some dissolved substances, so the correction is to say it is safe to drink rather than chemically pure.
- Claiming filter beds remove dissolved salts; the correction is that filter beds remove insoluble solids, while dissolved substances remain.
- Omitting the source-selection step and starting at filtration; the correction is to state that an appropriate fresh water source is chosen first.
- Confusing sterilisation with filtration; the correction is that sterilisation kills microbes while filtration removes insoluble solids.
- Stating that chlorine removes dissolved salts; the correction is that chlorine kills microbes and does not remove dissolved substances.
- Naming only one sterilising agent when asked for examples; the correction is to include chlorine, ozone and ultraviolet light.
- Thinking desalination is always used; the correction is that it is required only when fresh water supplies are limited.
- Confusing desalination with filtration; the correction is that desalination removes dissolved salts, whereas filtration removes insoluble solids.
- Assuming sea water is already potable; the correction is that sea water contains high levels of dissolved salts and must be desalinated.
- Saying distillation removes salts by filtering; the correction is that distillation separates water by boiling and condensing, leaving dissolved salts behind.
- Describing reverse osmosis as simple filtration; the correction is that reverse osmosis uses pressure and a partially permeable membrane to retain dissolved ions.
- Claiming membrane processes remove all particles including water molecules; the correction is that water molecules pass through while most dissolved ions are retained.
- Thinking that all potable water treatment requires the same amount of energy; correction: treatment of fresh water is generally less energy-intensive than desalination of salty water.
- Confusing distillation with filtration; correction: distillation involves boiling and condensation and requires heating, whereas filtration separates insoluble solids without boiling.
- Assuming that energy is only needed for pumping water; correction: energy is also needed for heating, pressurising and operating treatment processes such as distillation and reverse osmosis.
- Confusing potable water with pure water; correction: potable water is safe to drink but may contain dissolved substances, whereas pure water contains only H₂O molecules.
- Thinking that filtration removes dissolved salts; correction: filtration removes insoluble particles, while dissolved salts require desalination.
- Believing that chlorination removes dirt; correction: chlorination kills microorganisms, and filtration removes insoluble particles.
- Using a pH meter without calibrating it; correction: calibrate the pH meter with buffer solutions before use to obtain reliable readings.
- Heating the distillation flask to dryness; correction: stop heating before the sample evaporates completely to avoid damage to the apparatus and potential hazards.
- Assuming the distillate is pure water because it turns anhydrous copper(II) sulfate blue; correction: anhydrous copper(II) sulfate only tests for the presence of water, not its purity. To confirm purity, check that the boiling point is exactly 100°C or evaporate to confirm no residue remains.