Required practical 13 · Chemistry · Chemistry Paper 2, both tiers
Analysing and purifying water
Analyse and purify water samples from different sources, including measuring pH, the mass of dissolved solids and distillation.
The exam trap. Potable is safe to drink; pure means one substance. Clear water is not proof of either.
Variables
- Independent (you change)
- The water sample (tap, rain, stream, salt water)
- Dependent (you measure)
- pH and mass of dissolved solids per unit volume
- Control (you keep the same)
- The same volume of each sample
- The same balance and evaporating basin method
- The same pH method
Apparatus
- Universal indicator paper or a pH meter
- Evaporating basin and balance
- Bunsen burner or water bath
- Measuring cylinder
- Distillation apparatus (flask, condenser, thermometer, collecting tube)
Method, and why each step matters
Measure the pH of each sample with a pH meter or universal indicator and colour chart.
Why pH shows whether the water is acidic, neutral or alkaline.
Weigh an empty evaporating basin, add a measured volume of sample and weigh again.
Why The difference gives the mass of water used.
Heat gently until all the water has evaporated, then reweigh the basin.
Why The increase in mass is the mass of dissolved solids.
Calculate mass of dissolved solids per 1000 g (or per dm³).
Why This allows samples of different sizes to be compared.
Distil salt water: heat it, condense the steam and collect the water.
Why Distillation separates water from dissolved salts.
Check the distillate: it should have pH 7, leave no solid and boil at 100 °C.
Why These show that the distillate is pure water.
Risks
| Hazard | How to reduce the risk |
|---|---|
| Hot apparatus and steam can burn. | Use tongs and heat gently. |
| Stream water may contain microorganisms. | Wear gloves and do not drink any sample. |
A worked set of results
| Sample | pH | Dissolved solids in g per 1000 g |
|---|---|---|
| Tap water | 7.5 | 0.3 |
| Rain water | 5.6 | 0.0 |
| Stream water | 6.8 | 3.0 |
| Salt water | 8.0 | 35 |
- Potable water can contain dissolved substances, so it is not chemically pure.
- Distillation needs a lot of energy, so it is used for drinking water only where fresh water is scarce.
Mistakes that cost marks
Assuming water is safe to drink because it is clear or distilled-looking.
Instead Clear is not safe: potable means safe to drink, pure means one substance.
Giving a pH result without saying how it was measured.
Instead Name the method: pH meter or universal indicator with a chart.
Heating the basin so fiercely that solid spits out.
Instead Heat gently, or finish on a water bath.