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Qualitative Analysis: Tests for Cations

Cation tests for aluminium, ammonium, calcium, copper(II), iron(II), iron(III) and zinc using aqueous NaOH and ammonia.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • describe the use of aqueous sodium hydroxide and/or aqueous ammonia to identify the following aqueous cations through the formation of precipitates (if any) and their subsequent solubility: aluminium, ammonium (together with evolution of ammonia gas upon warming), calcium, copper(II), iron(II), iron(III) and zinc (formulae of complex ions are not required)

O-Level cation identification uses the precipitate colour and its behaviour in excess aqueous sodium hydroxide or aqueous ammonia. Ammonium is the exception: warming with aqueous sodium hydroxide releases a gas.

1. Definition

A cation test uses aqueous sodium hydroxide and/or aqueous ammonia, together with the observed precipitate or gas result, to identify a positive ion in an aqueous sample.

2. Key Ideas

A. Core procedure

Use a fresh portion of the unknown solution for each reagent.

  1. Add aqueous sodium hydroxide dropwise and record any precipitate colour.
  2. Add aqueous sodium hydroxide in excess and record whether the precipitate dissolves.
  3. On a fresh portion, repeat with aqueous ammonia, dropwise and then in excess.
  4. For ammonium ions, add aqueous sodium hydroxide and warm; test the evolved gas with damp red litmus paper.
Write the full observation

“White precipitate” is incomplete when solubility in excess distinguishes the ion. Write, for example, “white precipitate forms and dissolves in excess aqueous sodium hydroxide to give a colourless solution”.

B. Required cation results

CationAqueous sodium hydroxideAqueous ammonia
Al³⁺white precipitate; soluble in excess, giving a colourless solutionwhite precipitate; insoluble in excess
NH₄ +on warming, ammonia gas is produced—
Ca²⁺white precipitate; insoluble in excessno precipitate
Cu²⁺light blue precipitate; insoluble in excesslight blue precipitate; soluble in excess, giving a dark blue solution
Fe²⁺green precipitate; insoluble in excessgreen precipitate; insoluble in excess
Fe³⁺red-brown precipitate; insoluble in excessred-brown precipitate; insoluble in excess
Zn²⁺white precipitate; soluble in excess, giving a colourless solutionwhite precipitate; soluble in excess, giving a colourless solution

These seven ions are the cations named in the O-Level qualitative-analysis syllabus. Flame tests and lead(II) tests are not part of this core table.

C. Patterns that reduce memorisation

  • Copper(II), iron(II), iron(III): distinctive coloured precipitates.
  • Aluminium vs zinc: both dissolve in excess NaOH(aq); only the zinc precipitate dissolves in excess NH₃(aq).
  • Calcium: white precipitate with NaOH(aq), but no precipitate with NH₃(aq).
  • Ammonium: gas on warming with NaOH(aq); confirm ammonia using damp red litmus paper.
Initial iron(II) observation

Record the initial green precipitate for Fe²⁺. It may turn brown on standing in air, but that later change must not replace the initial observation.

3. Detailed Explanations

Macroscopic, particle and symbolic descriptions

For copper(II) ions with aqueous sodium hydroxide:

  • Macroscopic: a light blue precipitate forms and remains insoluble in excess reagent.
  • Particle level: aqueous copper(II) and hydroxide ions join to form an insoluble ionic lattice.
  • Symbolic:

Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)

The ionic equation balances two hydroxide ions for the 2 + copper ion and labels the precipitate as a solid.

For the ammonium-ion test:

NH₄ + (aq) + OH⁻(aq) → NH₃(g) + H₂O(l)

4. Common Mistakes

  • An ion name is an inference, not an observation.
  • “Soluble in excess” means the precipitate disappears after more reagent is added; it does not mean no precipitate formed initially.
  • A dark blue solution is the result for copper(II) in excess aqueous ammonia, not excess sodium hydroxide.
  • Calcium gives no precipitate with aqueous ammonia in the syllabus table.
  • Ammonium produces ammonia only when aqueous sodium hydroxide is added and the mixture is warmed.
  • Do not identify gases by smelling them directly.

5. Exam Tips

  • State the reagent, whether it was added dropwise or in excess, and the complete observation.
  • Use fresh portions when comparing sodium hydroxide and ammonia results.
  • Learn comparison pairs: aluminium/zinc, iron(II)/iron(III), and calcium/aluminium.
  • Connect anion evidence using Tests for Anions and practise the answer structure in QA Method.

6. Worked Examples

Modelled example 1

copper(II)

Core

Problem

A light blue precipitate forms with aqueous sodium hydroxide and aqueous ammonia. It is insoluble in excess sodium hydroxide but dissolves in excess ammonia to give a dark blue solution. Identify the cation.
Study the worked solution

  1. Use precipitate colour

    Method

    Record a light blue precipitate with both reagents.

    Reason

    Colour narrows the prescribed cation possibilities.

    Working

    Initial observation: light blue precipitate.

  2. Use excess-reagent behaviour

    Method

    Identify copper(II) ions.

    Reason

    The precipitate is insoluble in excess NaOH but dissolves in excess NH₃ to form a dark blue solution.

    Working

    Cation: Cu²⁺.

Guided practice 2

aluminium or zinc

About 6 min

Problem

A white precipitate dissolves in excess aqueous sodium hydroxide. What test with aqueous ammonia distinguishes aluminium ions from zinc ions?

Compare excess-ammonia behaviour on fresh portions

Aluminium precipitate in excess NH3
Zinc precipitate in excess NH3

Hints

Hint 1: fresh portion
Use a fresh portion so sodium hydroxide from the first test does not affect the ammonia test.
Hint 2: contrast
Both may form white precipitates; the distinction appears in excess ammonia.
View solution step by step

  1. Apply ammonia correctly

    Method

    Add aqueous ammonia dropwise, then in excess, to a fresh portion.

    Reason

    Fresh material preserves the prescribed reagent conditions.

    Working

    Separate ammonia test on a fresh portion.

  2. Distinguish the ions

    Method

    Assign insoluble to aluminium and soluble to zinc.

    Reason

    Aluminium hydroxide remains insoluble in excess ammonia; the zinc precipitate dissolves to a colourless solution.

    Working

    Insoluble → Al³⁺; dissolves → Zn²⁺.

Common misconception 3

ammonium

Find and correct the mistake

Learner method

A learner adds aqueous sodium hydroxide at room temperature and writes “a gas is formed, so ammonium is present”. Correct the procedure and positive result.

Complete reagent, condition and gas test

Required condition
Positive gas result

View solution step by step

  1. Complete the test

    Method

    Add aqueous sodium hydroxide and warm.

    Reason

    Warming releases ammonia from ammonium ions under alkaline conditions.

    Working

    Unknown + NaOH(aq); warm.

  2. Confirm the gas

    Method

    Test with damp red litmus and observe it turn blue.

    Reason

    This confirms the evolved gas is alkaline ammonia rather than an unspecified gas.

    Working

    NH₃ evolved → NH₄ + supported.

Challenge 4

full salt evidence

Minimal support

Full-salt transfer

An unknown gives a green precipitate insoluble in excess aqueous sodium hydroxide. A fresh portion acidified with dilute nitric acid gives a white precipitate with aqueous barium nitrate. State the ions supported.

Interpret independent fresh-portion evidence

Cation
Anion

Hints

Hint 1: cation colour
Use the prescribed sodium-hydroxide precipitate colour table.
Hint 2: anion reagent
Barium ions form a white insoluble salt with sulfate ions after acidification.
View solution step by step

  1. Infer the cation

    Method

    Identify iron(II) ions from the green precipitate.

    Reason

    This is the prescribed Fe²⁺ observation with aqueous sodium hydroxide.

    Working

    Cation: Fe²⁺.

  2. Infer the anion

    Method

    Identify sulfate ions from the acidified barium-nitrate white precipitate.

    Reason

    White barium sulfate forms in the prescribed sulfate test.

    Working

    Anion: SO₄²⁻; evidence is consistent with iron(II) sulfate.

7. Mind Stretchers

Mind stretcher 1: Resolve an incomplete white-precipitate resultExtension

A student adds aqueous sodium hydroxide to an unknown and reports only “a white precipitate forms”. Explain why the cation cannot yet be identified and plan the observations needed to distinguish aluminium, calcium and zinc ions.

Show Answer

The initial white precipitate is shared by several cations. Add sodium hydroxide in excess: calcium remains insoluble, while aluminium and zinc dissolve. On a fresh portion, add aqueous ammonia dropwise and then in excess: aluminium remains insoluble in excess, zinc dissolves, and calcium gives no precipitate.

8. Quiz

Quiz Time!

Practise the seven required cations, reagent conditions, observations and inferences.

Go to Cation Tests Quiz