AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Using resources › Using the Earth's resources and obtaining potable water
Practise Potable water. 11 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What potable water is, how it is produced in the UK from fresh water, and how salty water can be desalinated. This subtopic includes the water purification required practical (pH, dissolved solids and distillation), so expect method questions and calculations as well as 'give reasons' questions on Paper 2.
Key facts
Potable water = water that is safe to drink (low levels of dissolved salts and microbes)
Pure water = only H2O molecules; potable water is NOT pure
UK: choose a fresh water source → filter beds → sterilise
Desalination: distillation or reverse osmosis (membranes); both need large amounts of energy
Dissolved solids (g/dm3) = mass of residue (g) ÷ volume of sample (dm3)
1 dm3 = 1000 cm3
Pure water: pH 7, boils at 100 °C, leaves no residue
Notes
Potable water and pure water
Potable water is water that is safe to drink. It must have sufficiently low levels of dissolved salts and microbes.
Potable water is not pure in the chemical sense, because it contains dissolved substances. Pure water contains only water molecules, H2O.
The method used to produce potable water depends on the supplies of water available and on local conditions.
Producing potable water in the UK
diagram
Rain provides fresh water (low levels of dissolved substances). It collects in the ground (ground water) and in lakes and rivers.
Step 1: choose a suitable source of fresh water.
Step 2: pass the water through filter beds to remove solid particles.
Step 3: sterilise the water to kill microbes. Sterilising agents are chlorine, ozone or ultraviolet (UV) light.
Fresh water only needs filtering and sterilising.
Desalination
If fresh water is scarce, salty water or sea water can be desalinated (the dissolved salts are removed).
Distillation: the water is boiled and the steam is condensed, leaving the salts behind.
Reverse osmosis: salty water is forced at high pressure through a membrane that lets water through but not the salts.
Both processes need large amounts of energy, so desalination is expensive. It is used mainly in dry countries with little fresh water.
Required practical: analysing and purifying water
diagram
pH: use universal indicator paper and a colour chart, or a pH meter.
Dissolved solids: weigh an empty evaporating basin, add a measured volume of the water, heat gently until all the water has evaporated, then reweigh. Mass of solids = final mass − mass of empty basin.
Distillation: heat the salty water; the steam passes into a condenser (or a delivery tube leading into a test tube in iced water), where it condenses. Collect the liquid (the distillate).
The steam condenses in the cold tube; the salts stay behind.
Check that the distillate is pure: pH 7, no solid left when a sample is evaporated, and it boils at 100 °C.
How to answer each type of question
Describe the treatment of fresh water, with reasons
2 to 4 marksGrade 5
Name each step in order.
Say what each step does: filter beds remove solid particles; sterilising kills microbes.
Name a sterilising agent: chlorine, ozone or UV light.
Example. In the UK, most potable water is produced from the fresh water in lakes and rivers. Describe how the fresh water is treated to make it potable. Give a reason for each step. (4 marks)
Show the model answerHide the model answer
The water is passed through filter beds (1) to remove solid particles (1). It is then sterilised using chlorine / ozone / UV light (1) to kill microbes (1).
Explain the difference between potable and pure water
2 marksGrade 4
Say what pure water contains: only water molecules (one substance).
Say what potable water contains: dissolved substances, so it is a mixture, but it is safe to drink.
Example. A bottle of spring water has a label that lists the calcium, magnesium and sodium ions in the water. Explain why a chemist would say that the spring water is not pure. (2 marks)
Show the model answerHide the model answer
Pure water contains only water molecules / only one substance (1). The spring water also contains dissolved substances (calcium, magnesium and sodium ions), so it is a mixture (1).
Don’t lose marks
Writing that potable water is pure. It is safe to drink but contains dissolved substances.
Saying filter beds remove dissolved salts or kill microbes. They remove solid particles only.
Writing that chlorine 'cleans' the water. Say it sterilises the water, killing microbes.
Forgetting to convert cm3 to dm3 when calculating g/dm3.
Saying desalination is rarely used because it is 'difficult'. The key point is that it needs large amounts of energy, so it is expensive.
More tips
Memory tricks
Potable is linked to 'potion': something you drink.
UK recipe in three words: source, filter, sterilise.
Filter = solids out; sterilise = microbes killed.
Sense check: sea water has roughly 35 g of dissolved salts per dm3. If you get thousands, check your cm3 to dm3 conversion.
Exam technique
For reason questions, give the step AND what it does: 'filtration' alone rarely scores, but 'filter beds remove solid particles' does.
In practical questions, say what you measure and how: 'weigh the empty evaporating basin first' earns credit.
The best evidence that a distillate is pure water is that it boils at exactly 100 °C.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State what potable water meansPotable water is water that is safe to drink.
Grade 4
Explain why potable water is not pureIt contains dissolved substances, whereas pure water contains only water molecules.
Grade 4
List the steps in treating fresh waterChoose a suitable source of fresh water, pass it through filter beds, then sterilise it.
Grade 5
Give reasons for each treatment stepFilter beds remove solid particles; sterilising with chlorine, ozone or ultraviolet light kills microbes.
Grade 5
Name two methods of desalinationDistillation, and processes that use membranes such as reverse osmosis.