Identification of Ions and Gases: Revision Notes

Condensed for the final weeks. For the full explanation, use the Identification of Ions and Gases study guide.

Tier note: the tests themselves are all 0620 Core: syllabus 12.5 has no Supplement outcomes. The one item marked (0620 Extended, 5070 required) is a side explanation that uses the idea of a reducing agent (6.4 outcomes 10 and 12). 5070 has no tiers, so O Level candidates need all of it.

This topic is pure recall. Learn the reagent, observation and conclusion for each — all three are needed for full marks, since a correct colour change without naming the reagent used to produce it rarely scores in full.

Flame tests

Ion	Flame colour
Li⁺	Red
Na⁺	Yellow
K⁺	Lilac
Ca²⁺	Orange-red
Ba²⁺	Light green
Cu²⁺	Blue-green

Method: clean nichrome wire in concentrated HCl, dip in sample, hold in a blue Bunsen flame. Sodium and potassium compounds give no useful precipitate with NaOH or ammonia, which is exactly why the flame test exists — it’s often the only practical way to identify them.

Cation tests — with sodium hydroxide

Cation	Add NaOH(aq)	Excess NaOH
Cu²⁺	Light blue precipitate	Insoluble
Fe²⁺	Green precipitate, turning brown near the surface on standing	Insoluble
Fe³⁺	Red-brown precipitate	Insoluble
Zn²⁺	White precipitate	Dissolves — colourless solution
Al³⁺	White precipitate	Dissolves — colourless solution
Ca²⁺	White precipitate	Insoluble
Cr³⁺	Green precipitate	Dissolves in excess*
NH₄⁺	No precipitate; warm → ammonia gas	—

With ammonia solution instead: Fe²⁺ and Fe³⁺ give the same precipitates as with NaOH (insoluble in excess); Cu²⁺ gives a light blue precipitate that dissolves in excess, giving a dark blue solution; Zn²⁺ redissolves in excess, Al³⁺ does not. That is how you tell zinc from aluminium — a favourite question. Cr³⁺ behaves like Al³⁺ and Zn²⁺ with NaOH (precipitate dissolving in excess) but, like Al³⁺, stays insoluble in excess ammonia — so the excess stage of either reagent cannot separate chromium from aluminium. The precipitate colour does, with either reagent: chromium(III) gives a green precipitate, aluminium a white one. (*The exact colour of the solution formed when chromium(III) hydroxide redissolves is additional detail beyond the core observation, not required.)

Ca²⁺ gives no useful precipitate with ammonia at all — aqueous ammonia is too weak a base to precipitate the more soluble calcium hydroxide in any noticeable amount. That absence of a precipitate is itself the identifying observation, telling calcium apart from aluminium, chromium and zinc, which all give a precipitate with ammonia at the dropwise stage.

Anion tests

Anion	Test	Positive result
Carbonate CO₃²⁻	Add dilute acid	Effervescence; gas turns limewater milky
Chloride Cl⁻	Acidify with HNO₃, add AgNO₃	White precipitate
Bromide Br⁻	As above	Cream precipitate
Iodide I⁻	As above	Yellow precipitate
Sulfate SO₄²⁻	Acidify with HNO₃, add Ba(NO₃)₂	White precipitate
Nitrate NO₃⁻	Add NaOH + aluminium foil, warm	Ammonia gas produced
Sulfite SO₃²⁻	Add acidified potassium manganate(VII)	Purple solution decolourises

Acidify first in the halide and sulfate tests — otherwise carbonate present would also precipitate and give a false positive. The sulfite test and the sulfur dioxide gas test give the same result (acidified potassium manganate(VII) turns from purple to colourless). (0620 Extended, 5070 required) This is because both SO₃²⁻ and SO₂ are reducing agents. The sulfite test is done on the solution: the ions react with the manganate(VII) directly.

Gas tests

Gas	Test	Result
Hydrogen	Lit splint	Squeaky pop
Oxygen	Glowing splint	Relights
Carbon dioxide	Bubble through limewater	Turns milky
Ammonia	Damp red litmus	Turns blue
Chlorine	Damp litmus paper	Bleached white
Sulfur dioxide	Acidified aqueous potassium manganate(VII)	Purple → colourless

If a question describes a solid reacting (for example, a carbonate with acid) and asks for a gas test, you are testing the gas that the reaction gives off — not the solid directly. See the Identification of Ions and Gases study guide for the reasoning behind every table above.

Exam traps

• Give reagent and observation and conclusion — one alone rarely scores.
• Zn²⁺ and Al³⁺ both give white precipitates dissolving in excess NaOH; only ammonia solution separates them.
• Don’t forget to acidify before silver nitrate or barium nitrate.
• Chlorine bleaches litmus; ammonia turns it blue. Different observations.
• “Precipitate” not “solid”; state the colour precisely (cream, not “pale yellow”).
• Confusing chromium(III)’s and copper(II)’s behaviour in excess reagents: chromium(III) hydroxide dissolves in excess NaOH, but stays insoluble in excess ammonia. Copper(II) hydroxide behaves the opposite way — it stays insoluble in excess NaOH, but dissolves in excess ammonia to a dark blue solution.
• Expecting calcium to give a precipitate with ammonia — it doesn’t, and that absence is itself the identifying result.
• Mixing up sulfite (decolourises acidified KMnO₄) with sulfate (white precipitate with acidified barium nitrate) — different tests entirely, despite the similar names.

Self-test

1. A white precipitate forms with NaOH and dissolves in excess. Name two possible cations and how to distinguish them.
2. Which gas relights a glowing splint?
3. Why acidify before adding silver nitrate?
4. A cream precipitate forms with acidified silver nitrate. Which ion is present?
5. Give the flame colour for potassium and for calcium.
6. A green precipitate forms with NaOH, dissolving in excess, but staying insoluble in excess ammonia. Identify the cation.
7. Why does calcium give no precipitate with aqueous ammonia?

Answers: 1. Zn²⁺ or Al³⁺ — add ammonia solution instead: the zinc precipitate redissolves in excess, the aluminium one does not. 2. Oxygen. 3. To remove carbonate ions, which would otherwise form a precipitate and give a false positive. 4. Bromide, Br⁻. 5. Potassium = lilac; calcium = orange-red. 6. Chromium(III), Cr³⁺. 7. Aqueous ammonia is too weak a base to precipitate calcium hydroxide, which is comparatively soluble, in any noticeable amount.