Flame tests are used to identify metal cations (positive ions) based on the characteristic colour they produce in a Bunsen burner flame. Different metal ions emit different colours of light when heated because their electrons absorb energy and move to higher energy levels, then emit light of a specific wavelength as they fall back down.

Flame test method:

1. Clean a platinum or nichrome wire loop by dipping it in concentrated hydrochloric acid and holding it in a blue Bunsen flame until no colour is produced.
2. Dip the clean wire loop into the sample (or dampen with hydrochloric acid first to convert to chlorides which vaporise more easily).
3. Hold the wire loop in the edge of the blue Bunsen flame.
4. Observe and record the flame colour.

Adding sodium hydroxide (NaOH) solution to a solution containing a metal cation can produce a characteristic precipitate (insoluble solid). The colour of the precipitate helps identify the metal ion.

To distinguish between Ca²⁺, Mg²⁺, and Al³⁺ / Zn²⁺ (all give white precipitates):

• Add excess NaOH. Aluminium and zinc hydroxides dissolve in excess NaOH, but calcium and magnesium hydroxides do not.
• To distinguish Al³⁺ from Zn²⁺, add ammonia solution: Al(OH)₃ does not dissolve in excess ammonia, but Zn(OH)₂ does.

To test for a carbonate ion (CO₃²⁻):

1. Add dilute hydrochloric acid to the sample.
2. If a carbonate is present, it will fizz (effervesce) as carbon dioxide gas is produced.
3. To confirm CO₂, bubble the gas through limewater (calcium hydroxide solution). The limewater turns milky / cloudy white.

The limewater test: CO₂ reacts with calcium hydroxide to form insoluble calcium carbonate, which makes the solution appear milky.

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

To test for halide ions (Cl⁻, Br⁻, I⁻):

1. Add dilute nitric acid to the solution first (to remove any carbonate ions that would interfere).
2. Then add silver nitrate solution (AgNO₃).
3. Observe the colour of the precipitate formed.

Further confirmation: silver chloride dissolves in dilute ammonia solution. Silver bromide dissolves only in concentrated ammonia. Silver iodide does not dissolve in ammonia at all.

To test for a sulfate ion (SO₄²⁻):

1. Add dilute hydrochloric acid to the solution first (to remove any carbonate ions that would interfere).
2. Then add barium chloride solution (BaCl₂).
3. If sulfate is present, a white precipitate of barium sulfate forms.

The hydrochloric acid is added first to remove carbonate ions. Without this step, barium carbonate (also a white precipitate) could form and give a false positive result for sulfate.

Identifying gases is an important part of chemical analysis. Each common gas has a specific test.

Notes on gas tests:

• The hydrogen "pop test" works because hydrogen is flammable and explosive when mixed with air and ignited.
• The oxygen test relies on oxygen supporting combustion — the glowing splint relights because oxygen provides the gas needed for burning.
• Carbon dioxide turns limewater milky because it forms insoluble calcium carbonate with calcium hydroxide.
• Chlorine is an oxidising agent and a bleach — it first acts as an acid (turning blue litmus red) then bleaches the dye (turning it white).
• Ammonia is a base — it turns damp red litmus paper blue. The litmus must be damp because ammonia is an alkali that dissolves in water to form an alkaline solution.

To test for ammonium ions (NH₄⁺):

1. Add sodium hydroxide solution to the sample.
2. Warm the mixture gently.
3. If ammonium ions are present, ammonia gas is released.
4. Hold damp red litmus paper above the mixture — it turns blue.

When carrying out several ion tests on the same solution, the order matters:

1. Carbonate test first — adding acid to the sample. This will not interfere with later tests.
2. Sulfate test second — add HCl (already present from carbonate test) then BaCl₂.
3. Halide test last — add HNO₃ then AgNO₃. This must be done after the sulfate test because barium ions and silver ions would both form precipitates and interfere with each other.