A student has a mixture of three substances: chalk, potassium chloride, and water.
Look at some information about these substances:
| Substance | Melting point ($^\circ\text{C}$) | Boiling point ($^\circ\text{C}$) | Solubility in water |
|---|---|---|---|
| Chalk | 1339 | - | Insoluble |
| Potassium chloride | 770 | 1420 | Soluble |
| Water | 0 | 100 |
Describe how the student can separate the mixture to get pure samples of all three substances. Explain why each method of separation works.
The student then separates two solid substances, Y and Z. She wants to check if they are pure. She measures the melting points of four samples of solid Z. Look at her results:
| Sample | Melting point ($^\circ\text{C}$) |
|---|---|
| 1 | 121 |
| 2 | 115 |
| 3 | 114–118 |
| 4 | 122–125 |
The student knows that a pure sample of solid Z has a melting point of $122^\circ\text{C}$. She concludes that sample 4 is the purest sample of solid Z. Do the results support her conclusion? Explain your answer using evidence from the table.
46 exam-style questions on OCR GCSE Chemistry Purity and separating mixtures. Each one has a worked solution and a mark scheme showing where the marks go.