MrChemCoach

Tests for Ions and Gases

IGCSE · O Level · AS · A Level · CBSE
All labs

Gas

Salt in the tube

Cation

Anion

Reagent shelf

Tap a reagent to add it to the tube.

Cations

Anions

Heat

Test

Lab notebook

Written for Cambridge IGCSE 0620, O Level 5070, AS & A Level 9701 and CBSE by Ajay Shekhawat. © 2026 Mr ChemCoach EdTech Pvt Ltd.

For revisionTheory, method and exam tips

What this experiment shows

Qualitative analysis is identifying what is in an unknown salt, and Cambridge tests it every year. The routine is fixed: sodium hydroxide and then aqueous ammonia for the metal ion, a flame test if it is a Group I or II metal, and then a separate test for the anion. The precipitate colours - and whether a precipitate redissolves in excess - are what separate the ions from one another.

Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s), pale blue

What you need

  • Sodium hydroxide solution and aqueous ammonia
  • Dilute nitric acid and silver nitrate
  • Barium nitrate (many CBSE labs use dilute hydrochloric acid and barium chloride)
  • Dilute hydrochloric acid for carbonate and sulfite
  • Nichrome wire for flame tests
  • Limewater, damp red litmus paper and acidified potassium manganate(VII) for gases
  • Aluminium foil with sodium hydroxide for nitrate

How it is done

  1. Add sodium hydroxide a little at a time, then in excess, and note both stages.
  2. Repeat with aqueous ammonia in a fresh tube, again to excess.
  3. For a flame test, clean the wire in acid, dip in the sample and hold it in a blue flame.
  4. For halides: acidify with nitric acid, then add silver nitrate.
  5. For sulfate: acidify with nitric acid, then add barium nitrate.
  6. For carbonate and sulfite: add dilute hydrochloric acid and test the gas.

What you should see

  • Copper(II): pale blue precipitate, dissolving in excess ammonia to a deep blue solution.
  • Iron(II): green precipitate. Iron(III): red-brown precipitate.
  • Aluminium and zinc: white precipitate that dissolves in excess sodium hydroxide.
  • Chloride white, bromide cream, iodide yellow with silver nitrate.
  • Sulfate: white precipitate with barium nitrate. Carbonate: fizzes with acid, gas turns limewater milky.

Where marks are lost

  • Not adding the reagent in excess. Aluminium and zinc are told apart at that step.
  • Skipping the acid before silver nitrate, so carbonate gives a false white precipitate.
  • Using hydrochloric acid before silver nitrate. It adds chloride ions of its own.
  • Writing "a precipitate" with no colour, or the wrong shade.
  • Naming a gas without testing it, or calling a gas sulfur dioxide because limewater went milky: carbon dioxide does that too.

Testing the gas

Any gas given off is tested while it is coming off. In this lab a bung and delivery tube are fitted to the sample tube, and the gas is bubbled through limewater or acidified potassium manganate(VII), or collected in a dry tube and tested at the mouth with a lighted splint, a glowing splint or damp litmus. Switch to the Gases bench to test each gas from its own generator.

NH4+(aq) + OH−(aq) → NH3(g) + H2O(l): damp red litmus turns blue

Questions students ask

How do I tell aluminium from zinc?
Both give a white precipitate with sodium hydroxide that dissolves in excess. Use ammonia: the zinc precipitate dissolves in excess ammonia, the aluminium one does not.
Why acidify before testing for a halide?
To destroy any carbonate present. Carbonate would give a white precipitate with silver nitrate and be mistaken for chloride.
What does a flame test tell you?
The metal ion. Lithium red, sodium yellow, potassium lilac, calcium orange-red, copper(II) blue-green.

Try these next

Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the lab above: identify the salt, then test every gas it gives off with the same apparatus as the gas bench. A study aid; not affiliated with Cambridge University Press & Assessment.