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GCSE Chemistry · key term
A melting interval covering only a very small temperature range, characteristic of a pure substance.
Used by OCR
Paper 1 (Foundation), Paper 2 (Foundation), Paper 3 (Higher), Paper 4 (Higher)
Revise: Melting point and purityFor a pure substance, all parts of the sample have the same composition and structure. They therefore melt at the same characteristic temperature, allowing for a small experimental uncertainty. This gives a sharp melting point: the change from solid to liquid happens over a very narrow temperature range.
Do not call a sample pure merely because it melts. State that a pure substance has a sharp melting point, whereas an impure sample usually melts over a wider range.
If the question asks about identity as well as purity, say that a sharp melting point matching the reference value supports the proposed identity.
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