The ion colours and tests you must know for SPM

Qualitative analysis rewards memory more than any other part of SPM Chemistry. There is no working to show, a precipitate is either the right colour or it is not, so the marks go to students who have the colours and tests firmly memorised. This is the organised set worth learning, from ion colours through to the confirmatory tests. Keep the colours of ions, precipitates and flames reference beside you as you revise.

Ion colours in solution

Most ions are colourless in aqueous solution, so the coloured ones stand out and are easy marks once memorised:

• Copper(II), Cu²⁺, blue.
• Iron(II), Fe²⁺, green (pale green).
• Iron(III), Fe³⁺, yellow to brown.
• Colourless: Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺, Zn²⁺, Pb²⁺, NH₄⁺ and most anions.

A single colour observation often narrows a mystery solution to one or two candidates before you test anything.

Precipitate colours with sodium hydroxide

Adding sodium hydroxide solution to a cation gives a hydroxide precipitate. The colour, and whether it dissolves in excess, together identify the ion:

• Cu²⁺, blue precipitate, insoluble in excess.
• Fe²⁺, green precipitate, insoluble in excess.
• Fe³⁺, brown precipitate, insoluble in excess.
• Al³⁺, Zn²⁺, Pb²⁺, white precipitate, soluble in excess to give a colourless solution.
• Ca²⁺, Mg²⁺, white precipitate, insoluble in excess.

The dividing question is always “does it dissolve in excess NaOH?”, that separates the three white precipitates that dissolve (Al³⁺, Zn²⁺, Pb²⁺) from the two that do not (Ca²⁺, Mg²⁺).

Precipitate colours with ammonia solution

Ammonia solution also precipitates hydroxides, but the “soluble in excess” pattern differs, and that difference does the identifying work:

• Cu²⁺, blue precipitate, soluble in excess to give a dark blue solution.
• Al³⁺, white precipitate, insoluble in excess.
• Zn²⁺, white precipitate, soluble in excess to give a colourless solution.
• Pb²⁺, white precipitate, insoluble in excess.

Comparing the two reagents is powerful: Al³⁺ and Zn²⁺ both dissolve in excess NaOH, but only Zn²⁺ dissolves in excess ammonia, so the pair of tests tells them apart.

Flame test colours

Heat the sample on a clean wire in a hot flame and read the colour:

• Lithium, Li⁺, red.
• Sodium, Na⁺, yellow (golden yellow).
• Potassium, K⁺, lilac (purple).
• Calcium, Ca²⁺, brick red (orange-red).
• Barium, Ba²⁺, green.

Flame tests are the easiest way to confirm the reactive-metal cations that give white precipitates and are otherwise hard to separate.

Confirmatory tests for cations

• Ammonium, NH₄⁺, warm with sodium hydroxide solution; ammonia gas is released, which turns moist red litmus paper blue.
• Iron(II) vs iron(III), beyond the green/brown precipitate colours, Fe³⁺ gives a blood-red colour with potassium thiocyanate, while Fe²⁺ does not.

Anion tests

• Carbonate, CO₃²⁻, add dilute acid; effervescence of carbon dioxide, which turns limewater milky.
• Sulfate, SO₄²⁻, add dilute hydrochloric acid then barium chloride solution; a white precipitate of barium sulfate forms.
• Chloride, Cl⁻, add dilute nitric acid then silver nitrate solution; a white precipitate of silver chloride forms.
• Nitrate, NO₃⁻, the brown ring test: add iron(II) sulfate solution, then carefully add concentrated sulfuric acid down the side of the tube; a brown ring forms at the junction.

The acid is added first in the sulfate and chloride tests to remove carbonate, which would otherwise give a false white precipitate, a detail examiners like to test. The full method sits in the qualitative analysis of anions experiment, with the cation counterpart in qualitative analysis of cations.

How to memorise all this

Tables like these are memorised by active recall, not rereading. Make a two-column card, ion on one side, observation on the other, and quiz yourself both ways: given the ion, state the colour; given the observation, name the ion. Then practise on real analysis questions, where you narrow candidates test by test. The commonest exam slip is forgetting the “soluble in excess” behaviour, which is exactly what separates look-alike white precipitates. If your child knows the reactions but muddles the colours under pressure, structured drilling fixes it fast. Our teachers coach SPM Chemistry online, one-to-one in English from RM50 an hour, with a paid one-hour trial to start, working qualitative analysis until every colour and test is automatic.