Qualitative analysis: identifying cations and anions

Qualitative analysis asks a simple question with a careful answer: which ions are present in an unknown salt? In SPM it appears in acids, bases and salts and in the practical paper, and it rewards students who describe observations precisely rather than guessing. This guide gives you a reliable order of tests, the colours to memorise, and the exact wording examiners look for.

Two halves of the problem

Every salt is built from a cation (a positive ion, usually the metal or ammonium) and an anion (a negative ion such as chloride, sulfate, carbonate or nitrate). You identify each half with its own set of tests. Do not mix the two up: sodium hydroxide and ammonia solution test for cations, while dilute acids, barium chloride, silver nitrate and the brown ring test are for anions.

Identifying cations: the two-reagent method

The core method is to add two alkalis in turn to separate portions of the solution and watch the precipitate.

1. Add sodium hydroxide solution a little at a time, then in excess.
2. Add ammonia solution to a fresh portion, a little at a time, then in excess.

The colour of the precipitate and whether it dissolves in excess narrow the cation down quickly.

• Cu²⁺ gives a blue precipitate with both. It is insoluble in excess NaOH but dissolves in excess ammonia to a dark blue solution.
• Fe²⁺ gives a green precipitate; Fe³⁺ gives a brown precipitate. Both are insoluble in excess of either alkali.
• Pb²⁺, Al³⁺ and Zn²⁺ all give a white precipitate with NaOH that dissolves in excess NaOH. To separate them, use ammonia: with ammonia, Zn²⁺ dissolves in excess but Pb²⁺ and Al³⁺ do not.
• Mg²⁺ and Ca²⁺ give a white precipitate that is insoluble in excess NaOH.
• NH₄⁺ gives no precipitate; instead, warming the mixture with NaOH releases ammonia gas, which is pungent and turns moist red litmus paper blue.

Because several cations give a white precipitate, you finish with a confirmatory test. For example, add potassium iodide to a suspected Pb²⁺ solution: a yellow precipitate of lead(II) iodide confirms it, Pb²⁺ + 2I⁻ → PbI₂. For Fe³⁺, potassium thiocyanate gives a blood-red colour; for Fe²⁺, potassium hexacyanoferrate(III) gives a dark blue precipitate. You can drill these observations in the qualitative analysis of cations experiment, and the colours are collected in the colours of ions, precipitates and flames reference.

Identifying anions: acid first, then a specific reagent

Anion tests usually begin by adding a dilute acid, then a specific reagent.

• Carbonate, CO₃²⁻: add dilute acid; you see effervescence, and the gas turns limewater milky, confirming carbon dioxide. CO₃²⁻ + 2H⁺ → H₂O + CO₂.
• Chloride, Cl⁻: add dilute nitric acid, then silver nitrate solution; a white precipitate of silver chloride forms that dissolves in ammonia. Ag⁺ + Cl⁻ → AgCl.
• Sulfate, SO₄²⁻: add dilute hydrochloric acid, then barium chloride solution; a white precipitate of barium sulfate forms that is insoluble in the acid. Ba²⁺ + SO₄²⁻ → BaSO₄.
• Nitrate, NO₃⁻: use the brown ring test. Add iron(II) sulfate solution, then carefully pour concentrated sulfuric acid down the side of the tube; a brown ring forms where the layers meet. Handle concentrated acid with care.

The nitric acid before the chloride test and the hydrochloric acid before the sulfate test are not optional, they remove carbonate ions that would otherwise give a false white precipitate. Missing that step is a classic exam slip.

Gas tests you must know cold

Several tests end with a gas, so learn these confirmations exactly:

• Carbon dioxide turns limewater milky.
• Oxygen relights a glowing wooden splinter.
• Hydrogen burns with a squeaky “pop” when a lighted splinter is brought near.
• Ammonia is pungent and turns moist red litmus paper blue.
• Chlorine is greenish-yellow and bleaches moist litmus paper.

Worked example: an unknown white solid

Suppose a colourless solution gives a white precipitate with NaOH that dissolves in excess, a white precipitate with ammonia that does not dissolve in excess, and a white precipitate with barium chloride after adding dilute hydrochloric acid. The cation dissolving in excess NaOH but not in excess ammonia points to Al³⁺ or Pb²⁺; the anion result confirms sulfate. A confirmatory potassium iodide test (no yellow precipitate) rules out lead, so the salt is aluminium sulfate. Notice how each observation removes possibilities until one answer survives.

How to earn every mark

Examiners mark the observation and the inference separately, so write both: “White precipitate forms, soluble in excess sodium hydroxide” and then “therefore the cation may be Al³⁺, Pb²⁺ or Zn²⁺.” Name the colour, say whether a gas or precipitate formed, and state whether it dissolved in excess. Vague words like “changes” or “reacts” score nothing.

Qualitative analysis is memory-heavy but completely learnable, because the same dozen results appear again and again. Build a small table of cation and anion results, test yourself until the colours are automatic, and practise writing observations in full sentences. If the colours keep blurring together, that is a quick thing to fix with a teacher, our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial; see how it works if you would like the tests drilled against real Paper 3 questions.