8.7 Required practical: identifying ions

The practical

This is AQA required practical 7 and Edexcel core practical 9.6C, and it is one of OCR's practical activities. You use the tests from the previous lessons to identify the ions in unknown salts.

A logical order

Positive ion (metal or ammonium):

1. Flame test (for \mathrm{Li^+}, \mathrm{Na^+}, \mathrm{K^+}, \mathrm{Ca^{2+}}, \mathrm{Cu^{2+}}).
2. Dissolve the solid in water and add sodium hydroxide solution (for \mathrm{Al^{3+}}, \mathrm{Ca^{2+}}, \mathrm{Mg^{2+}}, \mathrm{Cu^{2+}}, \mathrm{Fe^{2+}}, \mathrm{Fe^{3+}}). Add excess if the precipitate is white.

Negative ion:

1. Add dilute acid: fizzing and limewater turning milky shows carbonate.
2. Add dilute nitric acid then silver nitrate: a white, cream or yellow precipitate shows chloride, bromide or iodide.
3. Add dilute hydrochloric acid then barium chloride: a white precipitate shows sulfate.

Use a fresh sample of the solution for each test.

Worked example

The solid is calcium chloride, CaCl₂.

Safety

• Silver nitrate stains skin; barium chloride is toxic; sodium hydroxide is corrosive. Wear eye protection and gloves where needed.
• Use only small amounts and dispose of solutions safely.