Earth and atmosphere · GCSE Chemistry
Potable water
GCSE Chemistry revision on potable water: screening, sedimentation, filtration, chlorination, desalination by distillation or reverse osmosis, and the required practical to test and purify water.
Potable means safe to drink, not necessarily pure H₂O. Fresh water: filter then sterilise (chlorine, ozone or UV). Salt water: distil or reverse osmosis — effective but energy-hungry.
The important bits
What you need to know
- 1
Potable water is water that is safe to drink. It contains small amounts of dissolved substances. Pure water in Chemistry is only H₂O, which is not the target for taps.
- 2
In the UK, most drinking water comes from fresh sources: rain collected in lakes, rivers and aquifers. Treatment: screening (remove solids), sedimentation/flocculation, filtration through sand, then sterilisation.
- 3
Sterilisation kills microbes: chlorine is the usual UK method; ozone or ultraviolet light are alternatives. Chlorine is a halogen (link to Group 7) and is toxic in excess, so the dose is controlled.
- 4
Distillation produces very pure water by boiling and condensing, which also desalinate seawater, but it needs a large energy input. That is why it is used where energy is cheap or fresh water is scarce.
- 5
Reverse osmosis pushes seawater through a membrane that lets water through but not salts. High pressure means high energy cost, and membranes need maintenance. Waste brine must be managed.
- 6
Required practical: test pH, test for dissolved solids by evaporating a known volume and weighing the residue, then purify by distillation and re-test boiling point (100 °C) and no residue.
- 7
Waste water (sewage) is treated by screening, sedimentation (sludge to anaerobic digesters, methane as a fuel), and aerobic biological treatment of effluent before discharge. This is not the same as making tap water, but papers compare the two.
- 8
Evaluation: desalination versus treating fresh water — desalination is independent of rainfall but costly in energy and carbon emissions if the electricity is from fossil fuels.
Quotations worth analysing
Short evidence. Real method.
“Potable water is not the same as chemically pure water.”
Tap water has dissolved minerals. Pure water is H₂O only, boiling point 100 °C at 1 atm, no residue on evaporation. Do not say tap water is pure.
“Filter, then chlorinate.”
Solids out first, then microbes killed. Chlorinating muddy water is less effective and wastes chlorine. Order matters in method marks.
“Seawater → distillation or reverse osmosis → potable water.”
Both remove dissolved salts that filtration cannot. Both need energy. Distillation is the school practical version of the industrial idea.
Go deeper
The required practical is a purity check, not a water board
Take a sample. Universal indicator or a probe for pH (potable is near 7, not strongly acidic). Evaporate 50 cm³ in a pre-weighed evaporating basin, dry, reweigh: mass of dissolved solids. Distil another portion: thermometer in the vapour should read 100 °C for pure water, and the distillate leaves no residue. You are showing that distillation separates H₂O from dissolved ions. You are not claiming that a school Liebig condenser is how London is supplied. If the boiling point is not 100 °C, the liquid is not pure water — it may still be potable if it was tap water with minerals. That distinction is the point of the opening definition.
Go deeper
Desalination is an energy question wearing a water costume
To distil seawater you must supply the latent heat of vaporisation — a huge energy cost per kilogram. Reverse osmosis uses high-pressure pumps instead of boiling, often less energy than distillation but still far more than treating river water. Countries with plenty of rain (UK) treat fresh water. Countries with little rain and cheap energy (or expensive necessity) desalinate. A six-mark compare: availability of fresh water, energy cost, carbon dioxide if fossil electricity, waste brine, reliability. Do not say distillation is “better because it is purer” without the energy penalty. Chlorine is not used to remove salt; it does not.
Go deeper
Sewage treatment recovers energy as well as cleaning water
Screening removes plastics and grit. Settlement tanks produce sludge, which anaerobic bacteria digest, making methane you can burn for heat and electricity on site. The liquid effluent is treated with aerobic bacteria that break down dissolved organic matter, then it is discharged. This protects rivers from deoxygenation. It is a different job from making potable water, which starts from relatively clean sources. If a question asks why we do not drink treated sewage effluent in the UK as routine tap water, say public acceptance, remaining dissolved substances, and that cheaper fresh sources exist — plus the extra treatment that would be needed. Some places do reuse water after much more thorough purification.
See the idea in action
A student tests seawater and then distils it. Seawater has pH about 8, leaves a white salt residue on evaporation, and boils above 100 °C. After distillation the liquid boils at 100 °C and leaves no residue: the dissolved NaCl and other ions stayed in the flask. Industrially the same idea (or reverse osmosis) makes potable water where fresh sources are scarce, but the energy cost is high. UK tap water is not made this way: it is filtered fresh water sterilised with chlorine, and it still contains some dissolved minerals, so it is potable rather than chemically pure.
Exam technique
Turn knowledge into marks
Define potable versus pure. For UK tap water: screen, filter, chlorinate. For seawater: distillation or reverse osmosis plus energy cost. In the practical, pH, residue on evaporation, then distillation and boiling point 100 °C.
Common mistakes
Do not give these marks away
- 01
Saying potable water is chemically pure, or that chlorine removes salt from seawater.
- 02
Describing UK tap-water treatment as distillation.
- 03
Forgetting an energy disadvantage when evaluating desalination.
How is most drinking water treated in the UK?
ABy distilling river water
BBy screening, filtering and sterilising (e.g. with chlorine)
CBy reverse osmosis of seawater only
DBy adding sulfur dioxide
Show the answer
By screening, filtering and sterilising (e.g. with chlorine). The UK uses fresh sources. Solids are removed, then microbes are killed. Distillation and reverse osmosis are for desalination and cost much more energy.
Quick questions
If this is the bit you searched
What is potable water GCSE Chemistry?
Water that is safe to drink. It is not the same as pure water, which is only H₂O. Potable water still contains some dissolved substances.
How is tap water treated in the UK?
Fresh water is screened, allowed to settle, filtered, then sterilised with chlorine (or ozone or UV) to kill microbes.
How can seawater be made potable?
Desalination by distillation (boil and condense) or reverse osmosis (pressure through a membrane). Both need a lot of energy.
How do you test if a sample is pure water?
It should boil at 100 °C (at standard pressure) and leave no residue when evaporated. pH should be 7. Tap water may fail the residue test and still be potable.