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Qualitative Analysis: Cations (Table)

Learn and apply Qualitative Analysis: Cations (Table) in the published Chemistry course sequence.

  • GCE A-Level H2 Chemistry 9476-2027
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Qualitative Analysis: Cations: Orientation

Cation tests in Paper 4 are not about “guessing ions”. They are about writing reagent → observation → inference, and using solubility in excess to separate similar-looking precipitates.

Use this with Paper 4 Skills: Planning, MMO, PDO, ACE and the Practical and QA (A Level) hub so method, data, and evaluation marks stay aligned.

Definitions (Must Know)

A. Precipitate

A precipitate is an insoluble solid that forms when two aqueous solutions are mixed.

B. In excess

In excess means enough reagent has been added that some remains after any reaction is complete (used to test whether a precipitate dissolves).

C. Amphoteric hydroxide

An amphoteric hydroxide reacts with both acids and bases. In QA this shows up as a precipitate that dissolves in excess NaOH(aq) (e.g. Al(OH)₃(s), Zn(OH)₂(s)).

D. Complex ion

A complex ion forms when a metal ion binds a ligand (e.g. NH₃). In QA, complex formation can dissolve a precipitate (e.g. Cu²⁺ in excess NH₃(aq) gives a deep blue solution).

Detailed Explanations

A. Observation → comparison → inference

Add aqueous sodium hydroxide to one portion and aqueous ammonia to a fresh portion, first dropwise and then in excess. Record precipitate colour before solubility. Use the complete pattern, not one observation.

B. Supplied 9476 cation table

CationWith NaOH(aq)With NH₃(aq)
Al³⁺white precipitate, soluble in excesswhite precipitate, insoluble in excess
NH₄ +ammonia produced on heating—
Ba²⁺no precipitate with pure reagentsno precipitate
Ca²⁺white precipitate at high [Ca²⁺]no precipitate
Cr³⁺grey-green precipitate, soluble in excess to a dark-green solutiongrey-green precipitate, insoluble in excess
Cu²⁺pale-blue precipitate, insoluble in excessblue precipitate, soluble in excess to a dark-blue solution
Fe²⁺green precipitate turning brown in air, insoluble in excessgreen precipitate turning brown in air, insoluble in excess
Fe³⁺red-brown precipitate, insoluble in excessred-brown precipitate, insoluble in excess
Mg²⁺white precipitate, insoluble in excesswhite precipitate, insoluble in excess
Mn²⁺off-white precipitate rapidly turning brown in air, insoluble in excessoff-white precipitate rapidly turning brown in air, insoluble in excess
Zn²⁺white precipitate, soluble in excesswhite precipitate, soluble in excess

C. Discriminating evidence

  • White precipitate soluble in excess NaOH: compare Al³⁺ and Zn²⁺ using excess ammonia.
  • Green or off-white precipitate turning brown: use the initial colour to distinguish Fe²⁺ from Mn²⁺.
  • Blue precipitate dissolving to a dark-blue solution in excess ammonia is the high-information Cu²⁺ pattern.
  • “No precipitate” does not identify one ion by itself; combine it with other supplied evidence.

Worked Examples

Modelled example 1

Identify Copper(II) from the Complete Test Pattern

Core

Problem

An unknown forms a pale-blue precipitate with NaOH(aq), insoluble in excess. On a fresh portion, NH₃(aq) gives a blue precipitate that dissolves in excess to form a dark-blue solution. Identify the cation.
Study the worked solution

  1. Record observations

    Method

    Separate precipitate colour, excess-solubility behaviour and final solution colour.

    Reason

    Each is an observation; naming the ion before recording them loses the evidence chain.

    Working

    NaOH: pale-blue precipitate, insoluble in excess; NH₃: blue precipitate → dark-blue solution in excess.

  2. Match the supplied pattern

    Method

    Identify Cu²⁺.

    Reason

    The dark-blue solution in excess ammonia is the discriminating result in the supplied table.

    Working

    Inference: copper(II) ions are present.

Guided practice 2

Distinguish Aluminium from Zinc

About 6 min

Problem

An unknown gives a white precipitate soluble in excess NaOH(aq). A fresh portion gives a white precipitate insoluble in excess NH₃(aq). Identify the cation and state the decisive evidence.

Try this before viewing the solution

Candidates after excess NaOH
Inference from excess NH₃

Hints

Hint 1: retain candidates
A white precipitate soluble in excess sodium hydroxide is not unique.
Hint 2: use fresh ammonia result
Compare how aluminium and zinc hydroxides behave in excess ammonia.
View solution step by step

  1. Use the sodium hydroxide result

    Method

    Retain Al³⁺ and Zn²⁺ as candidates.

    Reason

    Both form white precipitates that dissolve in excess sodium hydroxide.

    Working

    NaOH alone does not distinguish them.

  2. Use the discriminating reagent

    Method

    Identify Al³⁺ because the fresh ammonia precipitate is insoluble in excess.

    Reason

    A zinc precipitate would dissolve in excess ammonia.

    Working

    Inference: Al³⁺; Zn²⁺ excluded.

Common misconception 3

Correct “A Dark-Blue Precipitate Forms”

Find and correct the mistake

Learner observation

After adding excess ammonia to a copper(II) sample, a learner records “a dark-blue precipitate forms”. Correct the observation sequence.

Try this before viewing the solution

Initial observation
In excess ammonia

View solution step by step

  1. Describe dropwise addition

    Method

    Record formation of a blue precipitate.

    Reason

    The initial solid phase must be distinguished from the later dissolved species.

    Working

    Observation: blue precipitate forms.

  2. Describe excess reagent

    Method

    Record that the precipitate dissolves, producing a dark-blue solution.

    Reason

    Solubility in excess ammonia is part of the identification pattern.

    Working

    Blue precipitate → dark-blue solution.

Examiner practice 4

Distinguish Magnesium from Zinc

4 marks

Examination question

Describe how aqueous ammonia distinguishes separate solutions containing Mg²⁺ and Zn²⁺. Include observations dropwise and in excess. [4 marks]

Try this before viewing the solution

View solution step by step

  1. Apply reagent consistently

    1 mark

    Method

    Add aqueous ammonia dropwise, then in excess, to fresh portions of each solution.

    Reason

    Both the initial precipitate and excess-solubility behaviour are required.

    Working

    Same reagent sequence for both candidates.

  2. Record the common start

    1 mark

    Method

    State that each gives a white precipitate on dropwise addition.

    Reason

    Initial colour alone does not distinguish these ions.

    Working

    Mg²⁺: white; Zn²⁺: white.

  3. Use excess behaviour

    2 marks

    Method

    State that the magnesium precipitate remains insoluble, while the zinc precipitate dissolves to a colourless solution in excess ammonia.

    Reason

    The contrasting solubility is the discriminating evidence.

    Working

    Insoluble ⇒ Mg²⁺; soluble ⇒ Zn²⁺.

Challenge 5

Identify Ammonium Without a Precipitate

Minimal support

Gas-test transfer

An unknown solution gives no useful precipitate pattern. When warmed with aqueous sodium hydroxide, it releases a gas that turns damp red litmus paper blue. Identify the cation, separating the observation from the inference.

Try this before viewing the solution

Gas inference
Cation inference

Hints

Hint 1: gas test
The observation is a colour change of damp red litmus, not the gas name.
Hint 2: source ion
Recall which cation releases ammonia on warming with aqueous alkali.
View solution step by step

  1. State the observation

    Method

    Record gas evolution on warming and damp red litmus turning blue.

    Reason

    These are directly observed results; the ion name is an inference.

    Working

    Observation: gas evolved; damp red litmus → blue.

  2. Infer gas and cation

    Method

    Identify ammonia gas and hence ammonium ions.

    Reason

    Ammonia is alkaline and is released from ammonium ions by warm aqueous hydroxide.

    Working

    NH₄ + (aq) + OH-(aq) → NH₃(g) + H₂O(l).

Mind Stretchers

Mind stretcher 1Extension

An unknown forms a green precipitate with NaOH(aq). The precipitate turns brown on standing. Suggest the cation and explain the colour change.

Show Hint

Use both reagents and both excess-solubility results; a white precipitate alone is not enough to identify the ion.

Show Answer

Mark scheme:

  • Cation: Fe²⁺ (green Fe(OH)₂(s)).
  • On standing, Fe²⁺ is oxidised by oxygen in air to Fe³⁺, forming brown Fe(OH)₃(s).

Mind stretcher 2: Resolving two white-precipitate candidatesExtension

Question. An unknown gives a white precipitate soluble in excess sodium hydroxide. A fresh portion gives a white precipitate insoluble in excess ammonia. Deduce the cation and explain why zinc is excluded.

Show Hint

Use both reagents and both excess-solubility results; a white precipitate alone is not enough to identify the ion.

Show Answer

The cation is aluminium, Al³⁺. Both Al³⁺ and Zn²⁺ form white precipitates soluble in excess NaOH, but zinc hydroxide also dissolves in excess ammonia whereas aluminium hydroxide remains insoluble. The second result excludes Zn²⁺.