Describe the test and observation to distinguish Al^3+ from Mg^2+. [2 marks]

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How are common cations, anions and gases identified by simple chemical tests in qualitative analysis?

Describe the chemical tests used in qualitative analysis to identify common cations (including with NaOH and aqueous ammonia), anions (carbonate, sulfate, halides, nitrate) and gases, and interpret the observations

A focused answer to the H2 Chemistry learning outcome on qualitative analysis. Tests for common cations with sodium hydroxide and aqueous ammonia, tests for carbonate, sulfate, halide and nitrate anions, and the standard gas tests, with the observations expected in Paper 4.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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The answer
Examples in context
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What this dot point is asking

SEAB wants you to describe the chemical tests used in qualitative analysis to identify common cations (with sodium hydroxide and aqueous ammonia), anions (carbonate, sulfate, halides, nitrate) and gases, and to interpret the observations. These tests are central to Paper 4 (practical) and recur in written structure-determination questions.

The answer

Cation tests with sodium hydroxide

Add sodium hydroxide dropwise then in excess; note the precipitate colour and whether it dissolves in excess:

Cation	With a little NaOH	In excess NaOH
$\text{Cu}^{2+}$	pale blue precipitate	insoluble
$\text{Fe}^{2+}$	green precipitate	insoluble
$\text{Fe}^{3+}$	red-brown precipitate	insoluble
$\text{Al}^{3+}$	white precipitate	dissolves (amphoteric)
$\text{Ca}^{2+}$	white precipitate (faint)	insoluble
$\text{NH}_4^+$	no precipitate; ammonia gas on warming	-

Cation tests with aqueous ammonia

Add aqueous ammonia dropwise then in excess:

$\text{Cu}^{2+}$ : pale blue precipitate that dissolves in excess to a deep blue solution ( $[\text{Cu}(\text{NH}_3)_4]^{2+}$ ).
$\text{Al}^{3+}$ : white precipitate, insoluble in excess (distinguishes it from the NaOH behaviour).
$\text{Fe}^{2+}$ , $\text{Fe}^{3+}$ : green and red-brown precipitates respectively, insoluble in excess.

Anion tests

Carbonate ( $\text{CO}_3^{2-}$ ): add dilute acid; effervescence of $\text{CO}_2$ that turns limewater milky.
Sulfate ( $\text{SO}_4^{2-}$ ): add acidified barium nitrate (or chloride); a white precipitate of $\text{BaSO}_4$ .
Halides ( $\text{Cl}^-$ , $\text{Br}^-$ , $\text{I}^-$ ): add acidified silver nitrate; white (Cl), cream (Br) or yellow (I) precipitate, confirmed by solubility in ammonia.
Nitrate ( $\text{NO}_3^-$ ): warm with sodium hydroxide and aluminium (or Devarda's alloy); the nitrate is reduced to ammonia, identified by a pungent gas turning damp red litmus blue.

Gas tests

Gas	Test and observation
carbon dioxide	turns limewater milky
hydrogen	lit splint gives a squeaky pop
oxygen	relights a glowing splint
ammonia	pungent; turns damp red litmus blue
chlorine	bleaches damp litmus (pale green gas)
sulfur dioxide	turns acidified potassium dichromate(VI) from orange to green

Worked example

An unknown solution gives a green precipitate with sodium hydroxide that is insoluble in excess and slowly turns brown on standing. With acidified silver nitrate it gives a cream precipitate that dissolves only in concentrated ammonia. Identify the salt.

Cation: a green precipitate with NaOH, insoluble in excess, that turns brown on standing (as $\text{Fe}^{2+}$ is oxidised to $\text{Fe(OH)}_3$ ) identifies iron(II), $\text{Fe}^{2+}$ .

\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe(OH)}_2 \text{ (green)}

Anion: the cream precipitate with acidified silver nitrate, soluble only in concentrated ammonia, identifies bromide, $\text{Br}^-$ .

\text{Ag}^+ + \text{Br}^- \rightarrow \text{AgBr} \text{ (cream)}

So the salt is iron(II) bromide, $\text{FeBr}_2$ .

Try it: Qualitative analysis helper - enter your observations with NaOH, ammonia, barium and silver tests to identify the ions.

Examples in context

Example 1. Identifying an unknown salt in Paper 4. The practical paper routinely supplies an unknown solid and asks candidates to identify its ions through a sequence of tests, recording each observation and deduction. Methodically working through the cation tests (NaOH then ammonia) and the anion tests is exactly the skill being assessed, and full observation recording earns the marks.

Example 2. Confirming a product after synthesis. After preparing a salt, a chemist confirms its identity by testing for the expected cation and anion. For instance, confirming copper(II) sulfate means showing both the deep blue ammonia complex (for $\text{Cu}^{2+}$ ) and a white barium precipitate (for $\text{SO}_4^{2-}$ ), tying qualitative analysis to preparative chemistry.

Try this

Q1. Describe the test and observation to distinguish $\text{Al}^{3+}$ from $\text{Mg}^{2+}$ . [2 marks]

Cue. Add excess NaOH: the $\text{Al(OH)}_3$ precipitate dissolves (amphoteric); the $\text{Mg(OH)}_2$ precipitate does not.

Q2. State the reagent and observation for the test for sulfate ions. [2 marks]

Cue. Acidified barium nitrate (or chloride); a white precipitate of $\text{BaSO}_4$ .

Q3. Describe how you would test a gas to confirm it is ammonia. [1 mark]

Cue. Hold damp red litmus paper in the gas; it turns blue (and the gas is pungent).

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Specimen (9729)5 marksAn aqueous solution of an unknown salt gives a pale blue precipitate with a little sodium hydroxide that does not dissolve in excess. With aqueous ammonia it gives a pale blue precipitate that dissolves in excess to a deep blue solution. With acidified barium nitrate it gives a white precipitate. Identify the cation and the anion present and write ionic equations.

Show worked answer →

Cation: the pale blue precipitate with NaOH (insoluble in excess) and the pale blue precipitate with ammonia that dissolves in excess to a deep blue solution identifies copper(II), Cu2+.
Cu2+ + 2OH- -> Cu(OH)2 (pale blue precipitate)
With excess ammonia: Cu(OH)2 + 4NH3 -> [Cu(NH3)4]2+ + 2OH- (deep blue solution).

Anion: the white precipitate with acidified barium nitrate indicates sulfate, SO4^2-.
Ba2+ + SO4^2- -> BaSO4 (white precipitate).

So the salt is copper(II) sulfate.

Markers reward Cu2+ from the two precipitate behaviours with equations, SO4^2- from the barium test with equation, and the identity of the salt.

2023 (style)4 marksDescribe the chemical test, including reagent and observation, to identify (a) carbonate ions, (b) ammonium ions, and (c) the gas carbon dioxide.

Show worked answer →

(a) Carbonate ions: add dilute acid; effervescence occurs and the gas turns limewater milky (it is CO2).
CO3^2- + 2H+ -> H2O + CO2.

(b) Ammonium ions: warm with sodium hydroxide; a pungent gas (ammonia) is released that turns damp red litmus blue.
NH4+ + OH- -> NH3 + H2O.

(c) Carbon dioxide: bubble the gas through limewater; it turns the limewater milky (white precipitate of calcium carbonate).

Markers reward the reagent and observation for each, with equations where relevant.