A student has a sample of benzoic acid. It contains some sand and a small amount of salt.
| Substance | Solubility in cold water | Solubility in hot water |
|---|---|---|
| Benzoic acid | very low | high |
| Sand | insoluble | insoluble |
| Salt | high | high |
Which method gives the purest sample of benzoic acid?
Dissolve the sample in hot water, filter while hot, cool the filtrate, then filter to collect the crystals.
Dissolve the sample in cold water, filter, then evaporate the filtrate to dryness.
Dissolve the sample in hot water, filter while hot, then evaporate the filtrate to dryness.
Dissolve the sample in hot water, leave it to cool, then filter to collect the solid left on the filter paper.
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.