A student makes copper sulfate crystals from copper sulfate solution. The solution also contains a small amount of a soluble impurity.
The student evaporates only some of the water and then leaves the solution to cool, instead of evaporating all of the water.
Why does this method give purer crystals?
The soluble impurity stays dissolved in the remaining solution, so it is not left in the crystals.
The copper sulfate cannot react with the impurity unless all the water is evaporated.
The impurity evaporates with the water, so only copper sulfate is left behind.
More copper sulfate crystals form than if all the water were evaporated.
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.