Carry out and interpret qualitative analysis tests for common cations, anions and gases, describing the observations and the reagents used

What this dot point is asking

SEAB wants you to carry out and interpret the standard qualitative analysis tests: identifying metal cations using sodium hydroxide and ammonia solutions, identifying anions by their characteristic reactions, and identifying common gases by simple bench tests. The key skill is describing the observation precisely (colour, state, whether a precipitate dissolves) and concluding which ion or gas is present. This runs through the practical paper and the written papers.

The answer

Cation tests with sodium hydroxide

Adding sodium hydroxide solution to a solution of a metal salt forms a metal hydroxide precipitate whose colour, and whether it dissolves in excess alkali, identifies the cation:

• Copper(II) gives a blue precipitate, insoluble in excess.
• Iron(II) gives a green precipitate, insoluble in excess.
• Iron(III) gives a red-brown precipitate, insoluble in excess.
• Aluminium, zinc and lead(II) give a white precipitate that dissolves in excess sodium hydroxide.
• Calcium gives a white precipitate, insoluble in excess.
• Ammonium gives no precipitate, but on warming releases ammonia gas (see below).

Cation tests with aqueous ammonia

Aqueous ammonia distinguishes the white precipitates that sodium hydroxide cannot:

• Aluminium gives a white precipitate, insoluble in excess ammonia.
• Zinc gives a white precipitate that dissolves in excess ammonia.
• Copper(II) gives a blue precipitate that dissolves in excess ammonia to a deep blue solution.

So zinc and aluminium, identical with sodium hydroxide, are told apart by ammonia: zinc redissolves, aluminium does not.

Anion tests

• Carbonate: add dilute acid; effervescence of carbon dioxide (turns limewater milky).
• Chloride: add dilute nitric acid then silver nitrate; a white precipitate forms.
• Iodide: add silver nitrate; a yellow precipitate forms.
• Sulfate: add dilute nitric acid then barium nitrate; a white precipitate forms.
• Nitrate: add sodium hydroxide and aluminium foil and warm; ammonia gas is released (turns damp red litmus blue).

Gas tests

• Carbon dioxide: turns limewater milky.
• Oxygen: relights a glowing splint.
• Hydrogen: a lit splint gives a squeaky pop.
• Ammonia: turns damp red litmus paper blue (and has a sharp smell).
• Chlorine: bleaches damp litmus paper (turning it white).
• Sulfur dioxide: turns acidified potassium manganate(VII) from purple to colourless.

Examples in context

Example 1. Confirming the contents of a fertiliser. A fertiliser is suspected to contain ammonium sulfate. Warming with sodium hydroxide releases ammonia (damp red litmus turns blue), confirming the ammonium ion, while barium nitrate after nitric acid gives a white precipitate, confirming sulfate. The two tests together identify the compound.

Example 2. A practical paper unknown. In the practical exam you are given an unlabelled solid, told to dissolve it, and asked to identify both ions. A systematic approach (sodium hydroxide for the cation, then specific reagents for the anion) lets you reach a firm conclusion and write the formula of the salt from the two ions found.

Try this

Q1. State the test and result that identifies hydrogen gas. [2 marks]

• Cue. A lit splint held at the mouth of the test tube gives a squeaky pop.

Q2. A solution gives a red-brown precipitate with sodium hydroxide. Identify the cation. [1 mark]

• Cue. Iron(III), $\text{Fe}^{3+}$ (red-brown iron(III) hydroxide).

Q3. Describe how you would distinguish a solution containing chloride ions from one containing iodide ions. [3 marks]

• Cue. Add dilute nitric acid then silver nitrate to each; chloride gives a white precipitate, iodide gives a yellow precipitate, so the colour of the silver halide tells them apart.