Water draining from old mines can contain dissolved metal ions. An environmental scientist tests samples of mine water by adding sodium hydroxide solution drop by drop until it is in excess.
The table shows the results for three samples.
| Sample | Observation with a few drops of sodium hydroxide solution | Observation with excess sodium hydroxide solution |
|---|---|---|
| 1 | white precipitate | precipitate dissolves to form a colourless solution |
| 2 | white precipitate | precipitate does not dissolve |
| 3 | green precipitate | precipitate does not dissolve |
Name the metal ion in sample 1 and in sample 2.
Explain why the scientist needs to add excess sodium hydroxide solution to identify the ions in samples 1 and 2.
Sample 3 contains iron(II) ions. After the tube is left to stand in air, the top of the green precipitate turns brown. Suggest why.
Iron(III) ions are Fe3+\mathrm{Fe^{3+}}Fe3+ and hydroxide ions are OH−\mathrm{OH^{-}}OH−. Give the formula of iron(III) hydroxide.
Describe how a flame test could be used to distinguish between the metal ions in sample 1 and sample 2. Give the result for each sample.
35 exam-style questions on Edexcel GCSE Chemistry 10.1 Qualitative analysis: tests for ions, covering 10.1.1 Principles of ion tests and flame tests, 10.1.2 Tests for cations and for ammonia, 10.1.3 Tests for anions, 10.1.4 Identification of ions in unknown salts, and 10.1.5 Instrumental methods of analysis. Each one has a worked solution and a mark scheme showing where the marks go.