The disappearing cross is one of the most common rate experiments in IGCSE Chemistry Paper 6. Sodium thiosulfate, made up to 40 cm³ with water, reacts with dilute hydrochloric acid in a conical flask on a paper with a cross. The mixture turns cloudy, and you time how long until the cross can no longer be seen from above. Watch the set-up built step by step, then build it yourself and check it against the points the examiners look for.
Examiner reports used: 0620 June 2023 (Paper 62), March 2024 (Paper 62), June 2024, November 2024 (Paper 62), June 2025 (Papers 61 and 63).
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A conical flask standing on a paper with a cross, looked at from above. Same flask, same cross and same person for every run.
Measuring cylinders for the thiosulfate and the water, a 10 cm³ measuring cylinder for the acid. A burette would be more accurate; a volumetric pipette cannot measure changing volumes.
Thiosulfate + water always 40 cm³, same volume of the same acid, temperature checked with a thermometer before and after (water bath to control it).
Time / s in whole seconds from adding the acid until the cross disappears; a smooth curve of time against volume, and 1 / time for the relative rate.
More set-ups: all IGCSE chemistry Paper 6 set-up builders. Then practise in the apparatus, methods and planning trainer. The experiment, readings and wording on this page are our own.