Heat, cool, weigh, repeat. Stop only when two masses agree.
Heating a solid in a crucible comes up again and again in 9701 Paper 5: water of crystallisation, thermal decomposition, a metal reacting with sulfur. Here you find x in MgSO4·xH2O by heating it to constant mass. Watch the set-up built step by step, then build it yourself and check it against the points the examiners look for.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge AS & A Level Chemistry 9701 Paper 5. It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 9701/54 May/June 2026, (heating hydrated sodium carbonate to constant mass to find x) · official past papers
Practise next: 9701/51 May/June 2026, (thermal decomposition of calcium nitrate in a crucible) · official past papers
Practise next: 9701/52 October/November 2024, (a metal heated with sulfur in a crucible, and evaporating to constant mass) · official past papers
Examiner reports used: 9701 March 2024, June 2024, November 2024 and March 2026, with the June 2026 Paper 51 and 54 mark schemes (no June 2026 report yet).
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What a full-mark diagram shows
The apparatus
Crucible and lid in a pipeclay triangle on a tripod, heated by a Bunsen burner on a heat-proof mat. Not a gauze.
Heating
Lid slightly ajar; heat gently at first so no solid spits out, then strongly.
Constant mass
Cool in a desiccator, weigh, then reheat, cool and reweigh until two masses agree. Say mass, not weight.
The result
Water lost and anhydrous residue to moles; x as a whole number. A bigger sample cuts the percentage error.
Habits the examiner reports keep asking for
Name the pipeclay triangle. A tripod or gauze is not the answer.
Constant mass needs reheating and reweighing; cooling alone, or “constant weight”, is not credited.
Gentle heating stops spitting, not “burning” and not “even heating”.
To reduce percentage error, use more sample, not a different balance.
A fume hood is for a toxic gas such as NO2 or SO2; “harmful” or “flammable” scores nothing.