Qualitative Analysis: Gas Tests and Cation Tests You Must Word Correctly

 

By Zaheer | Updated 15 October 2026

Disclosure: I created the O Level Chemistry Lab 3 app mentioned at the end and sell it on Gumroad. Everything in this post is free to use without it. This is independent study material, not affiliated with Cambridge or any exam board.

Qualitative analysis questions are some of the most predictable marks in O Level and IGCSE Chemistry. You are given an unknown gas or solution, you describe a test, and you state what you would see. Students who know the chemistry still lose marks because of loose wording: "limewater turns white", or "the gas is carbon dioxide" when the question asked for an observation. This post gives you the standard tests, the wording that earns marks, and five questions to try.

Tests for common gases

Gas Test Positive result
HydrogenHold a lighted splint at the mouth of the tubeA squeaky pop
OxygenInsert a glowing splintThe splint relights
Carbon dioxideBubble the gas through limewaterThe limewater turns milky (cloudy)
AmmoniaHold damp red litmus paper in the gasThe litmus paper turns blue
ChlorineHold damp litmus paper in the gasThe litmus paper is bleached (turns white)

Tests for cations with sodium hydroxide

Add a few drops of sodium hydroxide solution, NaOH(aq), to a solution of the unknown, then add excess and note what happens to the precipitate.

Cation A few drops of NaOH(aq) Excess NaOH(aq)
Copper(II), Cu2+Light blue precipitatePrecipitate does not dissolve
Iron(II), Fe2+Green precipitatePrecipitate does not dissolve
Iron(III), Fe3+Red-brown precipitatePrecipitate does not dissolve
Zinc, Zn2+White precipitatePrecipitate dissolves, giving a colourless solution
Aluminium, Al3+White precipitatePrecipitate dissolves, giving a colourless solution
Calcium, Ca2+White precipitatePrecipitate does not dissolve

Ammonium ions: warm the solution with NaOH(aq). Ammonia gas is given off, and it turns damp red litmus paper blue.

Telling zinc and aluminium apart: they look identical with NaOH. Use aqueous ammonia instead. The zinc precipitate dissolves in excess ammonia solution, but the aluminium precipitate does not.

Six wording traps

  1. "Limewater turns white." Write "limewater turns milky or cloudy".
  2. Mixing up the splints. Hydrogen needs a lighted splint and gives a squeaky pop. Oxygen needs a glowing splint and relights it.
  3. Forgetting "damp" or the colour of the litmus paper. Ammonia turns damp red litmus blue. Dry paper will not show the change properly.
  4. Giving a conclusion when asked for an observation. "Carbon dioxide is produced" is a conclusion. An observation is "the gas turns limewater milky".
  5. Writing "solid" instead of "precipitate". When a solid forms in a solution, call it a precipitate and give its colour.
  6. Stopping after the first addition. For cations, always say what happens in excess: "white precipitate, which dissolves in excess to give a colourless solution".

Try these five questions

Write your answers before reading mine.

  1. A gas relights a glowing splint. Name the gas.
  2. Describe the test for carbon dioxide and the positive result.
  3. Solution X gives a light blue precipitate with sodium hydroxide solution. The precipitate does not dissolve in excess. Name the cation in X.
  4. Solution Y gives a white precipitate with sodium hydroxide solution, which dissolves in excess. With aqueous ammonia the white precipitate also dissolves in excess. Name the cation in Y.
  5. A student writes: "I saw carbon dioxide being made." Rewrite this as a correct observation for the limewater test.

Answers

  1. Oxygen.
  2. Bubble the gas through limewater. The limewater turns milky (cloudy) if the gas is carbon dioxide.
  3. Copper(II), Cu2+.
  4. Zinc, Zn2+. Both aluminium and zinc give a white precipitate that dissolves in excess sodium hydroxide, but only the zinc precipitate dissolves in excess aqueous ammonia.
  5. "The gas given off turned limewater milky." This describes what was seen, not what the gas is.

Quick checklist

  • I can state all five gas tests with the splint or litmus type.
  • I say "milky" or "cloudy" for limewater, never "white".
  • I give the colour of every precipitate and what happens in excess.
  • I write observations as what I see, not what I think it is.
  • I can separate zinc from aluminium using aqueous ammonia.

Practise more

My O Level Chemistry Lab 3 covers experimental techniques and qualitative analysis in an interactive study app: O Level Chemistry Lab 3: Experimental Techniques & Qualitative Analysis.

Always check the official syllabus and past-paper mark schemes from your exam board for the exact tests and wording they expect, as these can differ between syllabuses and years. No exam result is guaranteed.

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