A manufacturer makes crystals of compound XY for use in electronic components.
The composition of one crystal is shown in the table.
| Substance present | Percentage by mass |
|---|---|
| Compound XY | 99.8% |
| Uncombined element X | 0.2% |
Pure XY has an accepted melting point of 154 ∘C154\,^\circ\text{C}154∘C. The crystal began to melt at 151 ∘C151\,^\circ\text{C}151∘C and finished melting at 153 ∘C153\,^\circ\text{C}153∘C.
A treatment then removes all the uncombined element X without changing compound XY.
Explain why the crystal is not pure before treatment. Predict its melting behaviour after treatment and explain why the treated crystal can be pure even though it contains both X and Y.
113 exam-style questions on AQA GCSE Chemistry 8.1 Purity, formulations and chromatography, covering 8.1.1 Pure substances, 8.1.2 Formulations, and 8.1.3 Chromatography. Each one has a worked solution and a mark scheme showing where the marks go.