Chemical Tests for Ions and Gases: GCSE & IGCSE

The chemical tests for ions in GCSE and IGCSE Chemistry fall into four groups: flame tests for metal ions, sodium hydroxide for metal hydroxide precipitates, acid-then-reagent tests for carbonates, sulfates and halides, and gas tests for hydrogen, oxygen, carbon dioxide, chlorine and ammonia. Every mark in these questions comes from two things: the reagent you add and the exact colour you see.

Flame test colours

A flame test identifies a metal ion from the colour it gives a Bunsen flame. Dip a clean wire loop in the sample, hold it in the blue flame and note the colour.

• Lithium (Li⁺) — crimson (red)
• Sodium (Na⁺) — yellow
• Potassium (K⁺) — lilac
• Calcium (Ca²⁺) — orange-red (brick red)
• Copper (Cu²⁺) — blue-green

The usual trap is lithium and calcium, which are both red-ish. Write “crimson” for lithium and “orange-red” for calcium, and never just “red” for both. Flame tests also struggle with mixtures: sodium’s strong yellow can hide everything else.

Sodium hydroxide: the precipitate colours

Add a few drops of sodium hydroxide solution to a solution of the unknown. Most transition metal ions form an insoluble hydroxide, which appears as a coloured precipitate. This is where “iron(II) precipitate colour” questions come from.

• Copper(II), Cu²⁺ — blue precipitate
• Iron(II), Fe²⁺ — green precipitate
• Iron(III), Fe³⁺ — orange-brown (red-brown) precipitate
• Aluminium, Al³⁺ — white precipitate that dissolves in excess sodium hydroxide, giving a colourless solution
• Zinc, Zn²⁺ — white precipitate that also dissolves in excess sodium hydroxide
• Calcium, Ca²⁺ — white precipitate that does not dissolve in excess

Aluminium and zinc look identical with sodium hydroxide alone. Where a specification needs you to tell them apart, the follow-up is aqueous ammonia: the zinc precipitate dissolves in excess, the aluminium one does not. Read “dissolves in excess” carefully, because it is the whole point of the test.

Ammonium ions are different. Warm the sample with sodium hydroxide and ammonia gas is given off, which turns damp red litmus paper blue.

Ionic equations for the precipitates, with state symbols

Questions often ask for the ionic equation. Only the ions that actually join up appear, and the precipitate is always (s). If state symbols still catch you out, our guide to state symbols in chemistry covers the rules.

• Copper(II): Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)
• Iron(II): Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s)
• Iron(III): Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)
• Chloride with silver nitrate: Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
• Sulfate with barium chloride: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Tests for anions: carbonate, sulfate and halides

Carbonate. Add dilute acid. If it fizzes, bubble the gas through limewater. A carbonate gives carbon dioxide, and the limewater turns milky (cloudy).

Sulfate. Add dilute hydrochloric acid, then barium chloride solution. A white precipitate of barium sulfate means sulfate ions are present. This is the answer to “test for sulfate ions”, and you need both reagents to earn both marks.

Halides. Add dilute nitric acid, then silver nitrate solution. The precipitate colour names the halide:

• Chloride — white precipitate
• Bromide — cream precipitate
• Iodide — yellow precipitate

Cream versus yellow is the hardest call to make by eye, so learn the order: white, cream, yellow as you go down Group 7.

Why you acidify first

Acid is added first to remove ions that would give a false positive. Barium ions also form a white precipitate with carbonate ions, and silver ions form precipitates with carbonate and hydroxide ions. Without acid, a carbonate in your sample could be mistaken for a sulfate or a halide. The acid reacts the carbonate away as carbon dioxide before the test reagent goes in.

The choice of acid matters too. Use nitric acid for the halide test, because hydrochloric acid would add chloride ions of its own and give a white precipitate whatever the sample contained. Use hydrochloric acid for the sulfate test, because sulfuric acid would add sulfate. Naming the wrong acid is a classic way to lose a mark.

Tests for gases

• Hydrogen — a lit splint gives a squeaky pop.
• Oxygen — a glowing splint relights.
• Carbon dioxide — limewater turns milky: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).
• Chlorine — damp litmus paper is bleached white. It may turn red briefly first, because chlorine solution is acidic.
• Ammonia — damp red litmus paper turns blue.

Litmus tests need the paper to be damp, because the gas has to dissolve before it can change the colour. Say “damp” every time.

Which boards include which tests

Specifications differ, so always check yours. The flame tests, the hydroxide precipitates for copper, iron(II), iron(III) and aluminium, and the carbonate, sulfate and halide tests are the core set across the GCSE and IGCSE courses. Cambridge IGCSE Chemistry has the widest list, and is the course where you should expect ammonium ions, nitrate ions and the aluminium-versus-zinc distinction as well. For the nitrate test, Cambridge uses aluminium foil with sodium hydroxide solution, which gives ammonia on warming. In GCSE courses some of these are limited to the separate (triple) science papers, so check which of your papers they fall on.

Quick-reference list

• Flame: Li crimson, Na yellow, K lilac, Ca orange-red, Cu blue-green.
• NaOH: Cu²⁺ blue, Fe²⁺ green, Fe³⁺ orange-brown, Al³⁺ and Zn²⁺ white and dissolve in excess, Ca²⁺ white and does not.
• NH₄⁺: warm with NaOH, ammonia turns damp red litmus blue.
• CO₃²⁻: acid, then limewater goes milky.
• SO₄²⁻: HCl, then BaCl₂, white precipitate.
• Halides: HNO₃, then AgNO₃: white, cream, yellow.
• Gases: H₂ squeaky pop, O₂ relights a glowing splint, CO₂ limewater milky, Cl₂ bleaches damp litmus, NH₃ turns damp red litmus blue.

Practise the tests on real questions

These tests are pure recall, which makes them ideal for flashcards and short written answers where you must name the reagent, the observation and the conclusion. You can revise them in GCSE AQA identification of ions, identification of common gases and GCSE Edexcel tests for ions, or the Cambridge IGCSE ions and gases notes. Then test yourself with the subtopic worksheet on Exaim, where every written answer is marked against the board’s mark scheme so you can see exactly which observation you missed.