Pure aspirin melts at 136 ∘C136\,^\circ\mathrm{C}136∘C.
A student makes a sample of aspirin and measures its melting temperature. The student then purifies the sample by recrystallisation and measures its melting temperature again.
| Sample | Melting temperature (∘C^\circ\mathrm{C}∘C) |
|---|---|
| Before recrystallisation | 121 to 129 |
| After recrystallisation | 132 to 135 |
Which conclusion is best supported by the data?
Recrystallisation increased the purity of the sample, but the sample is still not pure.
The sample is now pure, because it melts within 4 ∘C4\,^\circ\mathrm{C}4∘C of 136 ∘C136\,^\circ\mathrm{C}136∘C.
Recrystallisation added impurities, because the melting temperature of the sample changed.
Both samples are equally impure, because both samples melt over a range of temperatures.
51 exam-style questions on OCR GCSE Chemistry 2.1 Purity and separating mixtures, covering 2.1.1 Pure substances and melting point data, 2.1.2 Relative formula mass and empirical formula, 2.1.3 Formulations as useful mixtures, 2.1.4 Techniques for separating mixtures, and 2.1.5 Chromatography and Rf values. Each one has a worked solution and a mark scheme showing where the marks go.