The key idea
Potable water is water that is safe to drink; it is not pure water.It is produced by treating fresh water to remove harmful substances.
Use the labels to explain the scientific relationship shown.
The bit that matters
Keep the idea tight, then use the worked example to practise the exact exam wording.
Potable water
Potable water is water that is safe to drink.It is not chemically pure — it contains dissolved mineral salts and gases that give it taste and are often beneficial.The UK obtains potable water from surface water (rivers, lakes) and ground water (aquifers).The key treatment steps are sedimentation, filtration through sand and gravel, and disinfection (chlorination or UV treatment).
Desalination
Seawater and highly brackish water can be made potable by desalination.The two main methods are distillation (heating water to evaporate it, then condensing the steam) and reverse osmosis (forcing water through a semi-permeable membrane under pressure).Both methods are highly effective but require large amounts of energy, making them expensive and used mainly in water-scarce regions.
Testing water purity
The purity of water can be assessed by measuring its boiling point (pure water boils at exactly 100°C at atmospheric pressure), its melting point (0°C), and electrical conductivity (pure water is a very poor conductor).The presence of dissolved ions raises the boiling point and increases conductivity.Flame tests and precipitation reactions can identify specific dissolved ions.
Sustainable use of water resources
Globally, water shortages are a significant challenge due to population growth, pollution and climate change.Reducing water usage, treating wastewater for reuse, and developing affordable desalination technologies are all important.Wastewater from homes and industry must be treated to remove biological and chemical waste before being returned to rivers or the sea.
Definitions to learn
Potable water
Water that is safe to drink; it may contain dissolved substances and is not the same as pure water.
Desalination
The removal of dissolved salts from seawater to produce potable water by distillation or reverse osmosis.
Filtration
The removal of insoluble particles from water by passing it through a porous medium such as sand.
Chlorination
The addition of chlorine to water to kill microorganisms and make it safe to drink.
Reverse osmosis
Forcing water through a semi-permeable membrane under high pressure to remove dissolved salts.
Describe how fresh water is treated to make it potable.
Sedimentation: particles settle out.
Filtration through sand removes finer particles and microorganisms.
Chlorination kills remaining bacteria and other pathogens.
The water is now potable — safe to drink.
Filtration removes particles; chlorination kills microorganisms.
For any water question, distinguish clearly between potable (safe to drink, with dissolved substances) and pure (only H₂O molecules).
Potable does not mean pure.Pure water contains only water molecules; potable water also contains dissolved salts and gases.
How to score full marks
- 1Potable ≠ pure — always state that potable water contains dissolved substances; pure water contains only H₂O.
- 2The three treatment stages are sedimentation, filtration, then chlorination — state them in order.
- 3Desalination is expensive because of HIGH ENERGY COSTS — always include this when evaluating its use.
Try these yourself
Open each answer only after you have explained the full chemical process.
1Define potable water.[2 marks]
- 1.State what makes water safe to drink, and what potable does NOT mean.
2State the three main stages of treating fresh water to make it potable.[3 marks]
- 1.Recall the order of treatment.
3Explain why fresh water from rivers requires treatment before drinking but water from aquifers may need less treatment.[3 marks]
- 1.Compare source contamination levels.
4Describe how seawater can be made potable and state one disadvantage.[2 marks]
- 1.Name the process and its limitation.
5State the test for water purity using boiling point.[2 marks]
- 1.Describe the expected result for pure water.
6Explain why adding excess chlorine to drinking water could be harmful.[2 marks]
- 1.Consider chlorine's chemical properties.
7Describe the process of distillation for producing pure water from a salt solution.[4 marks]
- 1.Outline heating, condensation and collection.
8Give two reasons why it is important to reduce water waste in areas where water resources are limited.[2 marks]
- 1.Think about environment and cost.