Explain the difference between pure water and potable water.
[2 marks]
Total for this question: 2
Chemistry required practicals
Water analysis & purification
AQA 4.10 · RP8
GCSE Chemistry (8462) · Required practical 8 — method, variables, the marks examiners report students losing.
Board and spec code confirmed against AQA 8462 · registry checked 2026-07-17How this checking works
Analyse water samples for pH and dissolved solids, and produce pure water from a sample by distillation.
Apparatus
- Water samples
- Universal indicator or a pH probe
- Evaporating basin and balance
- Bunsen burner
- Distillation apparatus (flask, condenser, thermometer)
Method
- 1Test the pH of each water sample with universal indicator or a pH probe.
- 2Weigh an empty evaporating basin, then add a measured volume of the water sample.
- 3Evaporate the water off and reweigh the basin to find the mass of dissolved solids.
- 4To purify a sample, distil it: heat to boiling so water vapour forms, condense it in the condenser, and collect the pure water; dissolved solids are left behind.
- 5Test the distilled water — it should be neutral and leave no residue on evaporation.
Results & processing
- Mass of dissolved solids = (mass of basin + residue) - (mass of empty basin).
- Distilled water leaves no residue (it is pure); tap water leaves a residue because it contains dissolved solids.
Where students lose marks
Confusing pure water with potable water.
Fix: Pure water contains only water molecules (H2O); potable water is safe to drink but still contains small amounts of dissolved substances.
Not subtracting the empty basin mass.
Fix: Dissolved solids = (basin + residue) - empty basin; forgetting the empty mass gives a wrong result.
Assuming tap water is pure.
Fix: Tap water leaves a residue when evaporated because it contains dissolved solids, so it is not pure.
Improve the method
- Evaporate to constant mass (reweigh until the reading stops changing) for an accurate residue mass.
- Use the same volume of each sample so the comparison is fair.
Try it — exam-style
Easy
ORIGINAL
Easy
ORIGINAL
An empty basin has mass 45.20 g. After evaporating a water sample, the basin and residue have mass 45.38 g. Calculate the mass of dissolved solids.
[1 mark]
Total for this question: 1
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
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The pure-vs-potable distinction and the residue calculation are easy marks once drilled — I make sure they're banked. Free intro call, then a free first lesson.