Qualitative analysis of salts

This page covers one Form 4 content standard: qualitative analysis of salts. Qualitative analysis is the detective work of identifying which cation and which anion a salt contains, using colour, precipitates, gases and confirmatory tests. It is a favourite of the practical assessment and Paper 2 structured questions, and it rewards a systematic approach far more than memorised luck.

What qualitative analysis is

Qualitative analysis identifies the ions present in a salt, it answers “what is in it”, not “how much”. A full identification names the cation (the positive metal or ammonium ion) and the anion (the negative ion such as chloride, sulfate, carbonate or nitrate). You reach the answer by observing colour, adding reagents to see precipitates, testing any gas given off, and finishing with a confirmatory test.

Cation tests: sodium hydroxide and ammonia

The main cation test adds sodium hydroxide solution to the salt solution and then, on a fresh sample, ammonia solution. You record the colour of the metal hydroxide precipitate and whether it dissolves in excess reagent.

• Cu²⁺, blue precipitate, insoluble in excess.
• Fe²⁺, green precipitate, insoluble in excess.
• Fe³⁺, brown precipitate, insoluble in excess.
• Al³⁺, Pb²⁺, Zn²⁺, white precipitate, soluble in excess sodium hydroxide.

To separate the three white ones, use ammonia solution: only Zn²⁺ dissolves in excess ammonia; Al³⁺ and Pb²⁺ do not. This two-reagent comparison is central to the standard.

Anion tests

Each anion has its own reagent and observation:

• Carbonate, CO₃²⁻, add dilute acid; carbon dioxide effervesces and turns limewater milky.
• Chloride, Cl⁻, add dilute nitric acid then silver nitrate solution; a white precipitate forms.
• Sulfate, SO₄²⁻, add dilute hydrochloric acid then barium chloride solution; a white precipitate forms.
• Nitrate, NO₃⁻, the brown ring test (with iron(II) sulfate and concentrated sulfuric acid) gives a brown ring.

The acid is added first in the chloride and sulfate tests to remove carbonate ions, which would otherwise give a false white precipitate.

Gas and confirmatory tests

Several gases confirm an ion or a decomposition product: carbon dioxide turns limewater milky; ammonia turns damp red litmus blue and smells pungent; oxygen relights a glowing splint; hydrogen gives a “pop” with a lighted splint. A confirmatory test clinches a tentative result, for example, adding potassium iodide to a lead(II) salt gives a yellow precipitate of lead(II) iodide.

Worked example

Question. A colourless salt solution is tested. (1) With sodium hydroxide solution it gives a white precipitate that dissolves in excess. (2) With ammonia solution it gives a white precipitate that does not dissolve in excess. (3) Adding dilute nitric acid then silver nitrate gives a white precipitate. Identify the cation and the anion, and name the salt.

Step 1, Interpret the sodium hydroxide result. A white precipitate soluble in excess NaOH narrows the cation to Al³⁺, Pb²⁺ or Zn²⁺.

Step 2, Interpret the ammonia result. The precipitate does not dissolve in excess ammonia, which rules out Zn²⁺. So the cation is Al³⁺ or Pb²⁺.

Step 3, Interpret the anion test. A white precipitate with silver nitrate (after nitric acid) shows the anion is chloride, Cl⁻.

Step 4, Combine. Lead(II) chloride is only slightly soluble, so a clear solution more likely contains the fully soluble aluminium salt. The most consistent identification is aluminium chloride, AlCl₃ (a confirmatory test, e.g. potassium iodide giving a yellow precipitate, would confirm lead if Pb²⁺ were present).

Answer. Cation Al³⁺, anion Cl⁻; the salt is aluminium chloride, AlCl₃.

Practice question

Question. A green salt solution gives a green precipitate with sodium hydroxide solution that is insoluble in excess. When dilute hydrochloric acid is added to the solid salt, a gas is released that turns limewater milky. Identify the cation and the anion, and name the salt.

Answer. A green precipitate insoluble in excess sodium hydroxide indicates the cation Fe²⁺ (iron(II)). A gas released with acid that turns limewater milky is carbon dioxide, so the anion is carbonate, CO₃²⁻. The salt is iron(II) carbonate, FeCO₃.

Exam tip

Read cation results as two pieces of information: the colour of the precipitate and whether it dissolves in excess, both are needed, and using sodium hydroxide and ammonia is what separates the three white precipitates (only Zn²⁺ dissolves in excess ammonia). In the chloride and sulfate anion tests, add the acid first to destroy carbonate ions, or you may report a false positive. State observations precisely (“a white precipitate that dissolves in excess”) rather than vaguely (“it reacts”), because the marks are for the specific observation.

A systematic routine

Treat every unknown salt the same way and the analysis becomes reliable. First note the colour of the solid and solution: a blue solution suggests Cu²⁺, green suggests Fe²⁺, and a colourless or white solid suggests ions such as Na⁺, Ca²⁺, Al³⁺, Zn²⁺ or Pb²⁺. Second, run the cation test with sodium hydroxide, then ammonia, recording colour and solubility in excess. Third, run the anion test appropriate to what you suspect, remembering to acidify first for chloride and sulfate. Fourth, use a gas test or confirmatory test to settle any remaining doubt. Writing your observations and deductions side by side, as a table, is exactly how the structured questions are marked.

Because so many marks here are for stating an observation correctly, practise the exact wording: “effervescence, and the gas turns limewater milky” for carbonate; “a brown precipitate, insoluble in excess” for iron(III); “a white precipitate, soluble in excess sodium hydroxide but insoluble in excess ammonia” for aluminium or lead. This is the same rule-with-evidence discipline you met at the very start of the syllabus, now applied to identifying ions.

Where this fits

This is content standard 6.7, the closing standard of the Acids, Bases and Salts chapter. It builds on the salts you learnt to prepare in standard 6.6 and on the ideas of precipitation and gases from across the chapter. Consolidate the tests with the chapter revision notes and drill identifications with the practice questions. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers build a single decision table for cations and anions, because a systematic routine is what turns qualitative analysis from guesswork into reliable marks.

Quick recap

• Cation test: sodium hydroxide then ammonia; note precipitate colour and solubility in excess (only Zn²⁺ dissolves in excess ammonia).
• Anion tests: carbonate → CO₂; chloride → white ppt with silver nitrate; sulfate → white ppt with barium chloride; nitrate → brown ring.
• Acidify first in the chloride and sulfate tests.
• Finish with a gas test or confirmatory test; state precise observations.