AQA GCSE Chemistry · Using Resources
Water purification required practical
Analyse and purify samples of water from different sources, including measuring pH and dissolved solids, and distillation.
You test water samples for pH and for how much solid is dissolved in them, then purify salty water by distillation. Pure water has a pH of 7, no dissolved solids and boils at 100 °C.
Equipment
- Water samples: tap water, sea (salt) water, distilled water
- Universal indicator paper or a pH meter
- Evaporating basin, balance (0.01 g) and a water bath or Bunsen
- Conical flask, delivery tube and boiling tube in a beaker of ice (or a Liebig condenser)
- Measuring cylinder and thermometer
Variables
- Independent you change
- The source of the water sample
- Dependent you measure
- pH and mass of dissolved solids
- Control you keep the same
- Same volume of each sample (50 cm³)
- Heat until all the water has evaporated
- Same balance
Method
- 1
Test the pH of each sample with universal indicator paper or a pH meter.
- 2
Weigh an empty evaporating basin. Add 50 cm³ of a sample.
- 3
Heat until all the water has evaporated, let it cool, and weigh the basin again.
- 4
The increase in mass is the mass of dissolved solids.
- 5
Distil the salt water: heat it in a flask so steam passes through a delivery tube into a cold tube, where it condenses.
- 6
Test the distilled water: its pH should be 7, it should leave no solid and it should boil at 100 °C.
Worked example
Results for 50 cm³ of each sample.
| Sample | pH | Mass of solids (g) | Solids per dm³ (g) |
|---|---|---|---|
| Tap water | 7.2 | 0.02 | 0.4 |
| Sea water | 8.1 | 1.75 | 35.0 |
| Distilled sea water | 7.0 | 0.00 | 0.0 |
Working
Mass of solids = mass of basin after − mass of empty basin
Per dm³: 1000 cm³ ÷ 50 cm³ = 20, so sea water: 1.75 × 20 = 35.0 g/dm³
Conclusion
Sea water has far more dissolved salt than tap water. Distillation removes the dissolved solids completely, giving pure water with a pH of 7. Distilling sea water is possible but uses a lot of energy, so it is expensive.
Risks
- Hot glassware and steamLet equipment cool; don’t touch the delivery tube.
- Evaporating basin can spit when nearly dryHeat gently and wear eye protection.
- Suck-back when heating stopsLift the delivery tube out of the collecting tube before you stop heating.
Common mistakes
- Saying potable water is pure. Drinking water is safe but still contains dissolved substances.
- Not cooling the collecting tube. Steam needs cooling to condense back to liquid.
- Weighing the basin while it’s still warm. Rising warm air can make the balance reading inaccurate.
Better results
- Use a pH meter instead of indicator paper for a more precise pH.
- Use a larger volume of sample so the mass of solids is easier to measure accurately.
- Use a Liebig condenser for more efficient cooling during distillation.
Launching November 2026
7 exam questions on this practical.
Practise the method, the variables, the maths and what the results mean, then revise the Using Resources flashcards. It opens to students soon: join the waitlist and we’ll email you once, when it does.