Lesson 1 of 4

Distinguish aqueous cations

Observe a few drops first, then add the reagent in excess.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

I can identify aqueous cations.Syllabus
Syllabus K326 / K328, 6(a). Identify aqueous cations

Distinguish aqueous cations

Observe a few drops first, then add the reagent in excess.

Make a guess. You are not marked.

Aqueous sodium hydroxide is added to a solution of iron(II) ions and a green precipitate forms. The tube is left open to the air. What happens to the precipitate?

It turns red-brown on standing, as air oxidises it
It stays green, as the precipitate is already formed
It dissolves to a colourless solution over time

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It turns red-brown on standing, as air oxidises it

Oxygen in the air oxidises iron(II) hydroxide to iron(III) hydroxide, which is red-brown. Mark schemes reward this observation.

Use a fresh small portion of the unknown for each reagent. Add aqueous sodium hydroxide or aqueous ammonia a little at a time, mix, and record any precipitate. Then add excess reagent and record whether that precipitate dissolves. A precipitate is an insoluble solid, not simply a coloured solution.

Cation observations
Cation	Aqueous NaOH	Aqueous ammonia
Al³⁺	White precipitate; dissolves in excess to a colourless solution	White precipitate; insoluble in excess
Ca²⁺	White precipitate; insoluble in excess	No precipitate
Cu²⁺	Light blue precipitate; insoluble in excess	Light blue precipitate; dissolves in excess to a dark blue solution
Fe²⁺	Green precipitate; insoluble in excess; turns red-brown on standing	Green precipitate; insoluble in excess; turns red-brown on standing
Fe³⁺	Red-brown precipitate; insoluble in excess	Red-brown precipitate; insoluble in excess
Zn²⁺	White precipitate; dissolves in excess to a colourless solution	White precipitate; dissolves in excess to a colourless solution
NH₄⁺	On warming, ammonia evolves; test with damp red litmus	Not a useful ammonium test: the reagent already contains ammonia

Most precipitates in this table are metal hydroxides. For example, Cu²⁺(aq) + 2OH^-(aq) -> Cu(OH)₂(s). The copper ion can give a clear blue solution before reaction and a cloudy light-blue precipitate afterwards; describe both colour and physical change. Complex-ion formulae for reactions with excess ammonia are not required.

Worked exampleDistinguish aluminium from zinc

An unknown forms a white precipitate soluble in excess NaOH. What next?

Try it first. Then show the worked stepsHide the worked steps

This result fits both Al³⁺ and Zn²⁺; it is not enough to identify either.
Test a fresh portion with aqueous ammonia, then excess.
A white precipitate remaining in excess supports aluminium; dissolution to a colourless solution supports zinc.

Answer

Use a second discriminating test, rather than repeating the same ambiguous observation.

Check your understanding

With aqueous ammonia, an unknown gives a light-blue precipitate that dissolves in excess to a dark-blue solution. Which cation is likely present?

Zn²⁺, since its precipitate also dissolves in excess aqueous ammonia
Fe²⁺, whose green precipitate looks blue at first in excess ammonia
Cu²⁺, which gives both the blue precipitate and the dark-blue solution

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Cu²⁺, which gives both the blue precipitate and the dark-blue solution

Copper(II) hydroxide is a light-blue precipitate, and it dissolves in excess ammonia to give a dark-blue solution. Record both stages as evidence.

Exam-style question

Aqueous ammonia is added a little at a time, until in excess, to a solution containing copper(II) ions. Describe what you would observe. [2]

Your answer

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Sort these

Aqueous sodium hydroxide is added to each cation until it is in excess. Sort the cations by what you see.

Ca2+
NH4+
Fe3+
Cu2+
Al3+
Zn2+
Fe2+

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Precipitate dissolves in excess: Al³⁺ / Zn²⁺

Precipitate stays in excess: Ca²⁺ / Fe³⁺ / Cu²⁺ / Fe²⁺

No precipitate; ammonia given off on warming: NH₄⁺