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GCSE Chemistry · key term
A consistent, defined observation used to stop timing, such as the moment a cross is no longer visible.
Used by OCR
Specification CM5.2i · Paper 2 (Foundation), Paper 4 (Higher)
Revise: Rate Calculation ArithmeticSome investigations measure the time taken to reach the same fixed endpoint rather than directly measuring a mass or volume. In the disappearing-cross experiment, for example, timing stops when enough sulfur precipitate has formed to obscure the cross. The endpoint is a consistent comparison point, not necessarily the end of the reaction.
Paper 1 (Foundation), Paper 2 (Foundation), Paper 3 (Higher), Paper 4 (Higher)
A measured point that is kept the same in every trial, such as the moment a cross disappears.
For a fixed endpoint, a shorter reaction time means a faster reaction. Compare , not the times themselves, when numerical relative rates are required.
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