NSWChemistrySyllabus dot point

Inquiry Question 1: How are the ions present in the environment identified and measured?

Conduct qualitative investigations to test for the presence in aqueous solutions of cations and anions using flame tests, precipitation reactions and complexation reactions

Q: What is copper with ammonia?

Add dilute NH_3 to a Cu^2+ solution; pale blue Cu(OH)_2 forms, then with excess ammonia it dissolves to give the deep blue tetraammine complex:

Q: What is iron with thiocyanate?

Add KSCN to a Fe^3+ solution; a deep blood-red complex forms:

Q: What is q1?

State the flame test colour for each of: Li^+, Na^+, K^+, Ca^2+, Cu^2+. [3 marks]

Q: What is q2?

A solution gives a white precipitate with acidified AgNO_3 that dissolves in dilute ammonia. Calculate the mass of chloride ion in 50 mL of the solution if 0.072 g of AgCl is recovered. [3 marks]

A focused answer to the HSC Chemistry Module 8 dot point on qualitative ion identification. Flame tests for group 1 and 2 cations, precipitation tests for transition metals and halides, complexation tests for copper, iron and silver, a structured systematic analysis, and worked HSC past exam questions.

Generated by Claude Opus 4.812 min answerUpdated 2026-06-22

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

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What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

NESA wants you to be able to identify a list of named cations ( $Ba^{2+}, Ca^{2+}, Mg^{2+}, Pb^{2+}, Ag^+, Cu^{2+}, Fe^{2+}, Fe^{3+}$ ) and anions (chloride, bromide, iodide, hydroxide, acetate, carbonate, sulfate, phosphate) by their colour, flame test, precipitation behaviour with named reagents, and complexation behaviour. You should also be able to sequence tests so that one ion does not interfere with another.

The answer

Flame tests (group 1 and 2 cations mostly)

Heat a clean platinum or nichrome wire in a Bunsen flame until it glows colourless. Dip it in concentrated HCl to clean residual ions, then in the unknown, and observe the flame colour.

Cation	Flame colour
$Li^+$	Carmine red
$Na^+$	Persistent yellow-orange
$K^+$	Lilac (use cobalt-blue glass to block sodium)
$Ca^{2+}$	Brick red
$Sr^{2+}$	Crimson
$Ba^{2+}$	Apple green
$Cu^{2+}$	Blue-green
$Pb^{2+}$	Pale blue (variable)

Flame tests are qualitative only. They are excellent for group 1 and group 2 cations because the electron transitions are in the visible range and the colours are characteristic.

Precipitation tests for cations

Add a named reagent and observe the precipitate (colour, texture, solubility in excess).

Reagent	$Cu^{2+}$	$Fe^{2+}$	$Fe^{3+}$	$Pb^{2+}$	$Ag^+$	$Mg^{2+}$	$Ba^{2+}$ , $Ca^{2+}$
Dilute NaOH	Blue gel	Dirty green	Rust brown	White (redissolves in excess)	Brown $Ag_2O$	White	No ppt (Ca slow)
Dilute $NH_3$	Blue, then deep blue with excess (complex)	Green, darkens	Brown	White	Brown then dissolves in excess	White	No ppt
$Na_2CO_3$	Green-blue	Green-white	Brown	White	Pale yellow	White	White
$K_2CrO_4$	Brown	(Pale)	(Brown)	Yellow $PbCrO_4$	Red $Ag_2CrO_4$	(No)	Yellow $BaCrO_4$ , faint $CaCrO_4$

The $Fe^{2+}$ green hydroxide darkens on standing as it oxidises in air to $Fe^{3+}$ brown. State that change explicitly if you see it in a question.

Precipitation tests for anions

Anion	Reagent	Observation
$Cl^-$	$AgNO_3$ in dilute $HNO_3$	White ppt of $AgCl$ , dissolves in dilute $NH_3$
$Br^-$	$AgNO_3$ in dilute $HNO_3$	Cream ppt of $AgBr$ , partly dissolves in concentrated $NH_3$
$I^-$	$AgNO_3$ in dilute $HNO_3$	Yellow ppt of $AgI$ , insoluble in $NH_3$
$SO_4^{2-}$	$BaCl_2$ in dilute $HCl$	White ppt of $BaSO_4$ , insoluble in acid
$CO_3^{2-}$	Dilute $HCl$	Effervescence of $CO_2$ , turns limewater milky
$PO_4^{3-}$	$AgNO_3$ in neutral solution	Yellow ppt of $Ag_3PO_4$
$OH^-$	Universal indicator or pH	Blue/purple, pH > 10
$CH_3COO^-$	Warm with conc. $H_2SO_4$ and ethanol	Fruity smell of ethyl ethanoate (ester)

The order halide colours (white, cream, yellow) and ammonia solubility (yes, partial, no) is the standard halide differentiation.

The acidification step (dilute $HNO_3$ for $AgNO_3$ , dilute $HCl$ for $BaCl_2$ ) destroys any carbonate, which would otherwise also precipitate and give a false positive.

Complexation tests

Complexation distinguishes ions that give similar precipitates by re-dissolving one in excess reagent through formation of a soluble complex ion.

Silver halides with ammonia is the canonical example. $AgCl$ dissolves in dilute $NH_3$ , $AgBr$ partly dissolves in concentrated $NH_3$ , $AgI$ does not dissolve:

AgCl_{(s)} + 2NH_{3(aq)} \rightarrow [Ag(NH_3)_2]^+_{(aq)} + Cl^-_{(aq)}

Copper with ammonia. Add dilute $NH_3$ to a $Cu^{2+}$ solution; pale blue $Cu(OH)_2$ forms, then with excess ammonia it dissolves to give the deep blue tetraammine complex:

Cu(OH)_{2(s)} + 4NH_{3(aq)} \rightarrow [Cu(NH_3)_4]^{2+}_{(aq)} + 2OH^-_{(aq)}

Iron(III) with thiocyanate. Add $KSCN$ to a $Fe^{3+}$ solution; a deep blood-red complex forms:

Fe^{3+}_{(aq)} + SCN^-_{(aq)} \rightarrow [FeSCN]^{2+}_{(aq)}

This test is so sensitive it picks up traces of $Fe^{3+}$ at sub-ppm levels.

Iron(III) with hydroxide vs iron(II) with hydroxide. $Fe^{3+}$ gives rust-brown $Fe(OH)_3$ ; $Fe^{2+}$ gives dirty green $Fe(OH)_2$ that browns on standing. Adding $KSCN$ confirms which is present, since only $Fe^{3+}$ gives the red colour.

A systematic procedure

When you do not know what is in the sample:

Look. Coloured solution suggests $Cu^{2+}$ (blue), $Fe^{3+}$ (yellow-brown), $Fe^{2+}$ (pale green), $CrO_4^{2-}$ (yellow), $MnO_4^-$ (purple).
Flame test on a small portion to screen group 1/2 cations.
Add NaOH to a fresh portion to test for transition metal hydroxides.
Targeted tests for suspected ions on fresh portions: $AgNO_3$ for halides, $BaCl_2$ for sulfate, dilute HCl for carbonate, $KSCN$ for $Fe^{3+}$ .
Always use a fresh portion for each test. Acidify with the appropriate acid to suppress interferences (carbonate is the most common false-positive).

Common traps

Confusing $Fe^{2+}$ and $Fe^{3+}$ hydroxides. Dirty green = $Fe^{2+}$ , rust brown = $Fe^{3+}$ . The $Fe^{2+}$ precipitate goes brown on standing.

Forgetting to acidify before $AgNO_3$ or $BaCl_2$ tests. Carbonate gives a false positive with both silver and barium. A drop of dilute acid before the reagent removes it.

Stating that copper hydroxide is soluble in ammonia in one step: It precipitates first (pale blue), then redissolves in excess to a deep blue complex. Examiners want both stages described.
Treating flame tests as quantitative: They are not. Two ions in the same flame mask each other (sodium is so bright it hides almost everything; use cobalt-blue glass).
Mixing test reagents into one tube: Each test gets a fresh sample portion. Sequential addition produces unreadable mixtures.

Examples in context

Example 1. Identifying contaminants in groundwater near Botany Bay. Following a 1990s legacy spill of dense non-aqueous phase liquids at the Botany Industrial Park, NSW EPA contractors run a qualitative ion screen on every new groundwater sample before quantitative GC-MS. The test sequence covers: flame test (no yellow $Na^+$ confirms no halite intrusion), acidified $AgNO_3$ (a white $AgCl$ precipitate indicates chloride), and acidified $BaCl_2$ (a white $BaSO_4$ flag for sulfate from acid mine drainage). A positive reaction triggers escalation to quantitative AAS for the cation profile. The HSC qualitative framework students apply at the bench is the same first-pass screen that scopes the contamination plume.

Example 2. NSW HSC depth study unknown solution identification. A common Stage 6 task gives students six unlabelled solutions and asks them to identify the cation and anion in each using only the qualitative test reagents on the bench. A solution that turns blue with NaOH (forming $Cu(OH)_2$ ) and then dissolves to deep blue with excess ammonia ( $[Cu(NH_3)_4]^{2+}$ ) is copper. The same solution, treated with $BaCl_2$ in acid and giving a white precipitate, contains sulfate. The student writes the two ion identification net ionic equations: $Cu^{2+} + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+}$ and $Ba^{2+} + SO_4^{2-} \rightarrow BaSO_4$ . NESA markers reward both the observation and the equation.

Try this

Q1. State the flame test colour for each of: $Li^+$ , $Na^+$ , $K^+$ , $Ca^{2+}$ , $Cu^{2+}$ . [3 marks]

Cue. $Li^+$ crimson; $Na^+$ yellow; $K^+$ lilac; $Ca^{2+}$ brick red; $Cu^{2+}$ blue-green.

Q2. A solution gives a white precipitate with acidified $AgNO_3$ that dissolves in dilute ammonia. Calculate the mass of chloride ion in 50 mL of the solution if 0.072 g of $AgCl$ is recovered. [3 marks]

Cue. $n(AgCl) = 0.072 / 143.32 = 5.02 \times 10^{-4}$ mol; $n(Cl^-) = 5.02 \times 10^{-4}$ mol; mass = $5.02 \times 10^{-4} \times 35.45 = 0.0178$ g.

Q3. A solution gives a pale-green precipitate with NaOH that darkens on standing. (a) Identify the cation. (b) Write the net ionic equation. (c) Describe one additional test to confirm. [2+1+2 marks]

Cue. (a) Iron(II), $Fe^{2+}$ . (b) $Fe^{2+} + 2OH^- \rightarrow Fe(OH)_2$ . (c) Oxidation to brown $Fe(OH)_3$ on air exposure or red colour with $SCN^-$ after oxidation.

Exam-style practice questions

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

2022 HSC5 marksAn unknown aqueous solution gives a brick-red flame test, a white precipitate with silver nitrate that dissolves in dilute ammonia, and no precipitate with sodium hydroxide. Identify the cation and the anion present. Write balanced ionic equations for the precipitation and the complexation steps.

Show worked answer →

A 5 mark answer needs both ions, both equations, and one sentence of reasoning per step.

Brick-red flame test points to calcium $Ca^{2+}$ . (Strontium is crimson, lithium is bright red but more carmine; brick-red is the standard description for calcium.)

No precipitate with NaOH rules out transition metal cations (which form coloured hydroxides) and rules out $Mg^{2+}$ (which gives white $Mg(OH)_2$ ). $Ca(OH)_2$ has $K_{sp} \approx 5 \times 10^{-6}$ and is moderately soluble, so dilute NaOH does not precipitate calcium. Consistent with $Ca^{2+}$ .

White precipitate with silver nitrate that dissolves in dilute ammonia is the classic test for chloride $Cl^-$ :

Ag^+_{(aq)} + Cl^-_{(aq)} \rightarrow AgCl_{(s)}

AgCl_{(s)} + 2NH_{3(aq)} \rightarrow [Ag(NH_3)_2]^+_{(aq)} + Cl^-_{(aq)}

The first equation is the precipitation; the second is the complexation that distinguishes chloride from bromide (which is cream and only partially dissolves in concentrated ammonia) and iodide (which is yellow and does not dissolve).

Conclusion. The unknown is calcium chloride, $CaCl_2$ .

Markers reward (1) calcium from the flame, (2) ruling out other cations using the hydroxide result, (3) chloride from the silver nitrate result, (4) the complexation equation, (5) using both pieces of evidence consistently.

2019 HSC4 marksDescribe a systematic procedure to identify which of the following ions are present in an unknown aqueous solution:

Pb^{2+}

Cu^{2+}

Fe^{3+}

Cl^-

SO_4^{2-}

Show worked answer →

A 4 mark answer needs a sequence of tests with observations and conclusions for each candidate.

Step 1: Inspect the colour: $Cu^{2+}$ is blue, $Fe^{3+}$ is yellow-brown. If the solution is colourless, both are absent. If blue, $Cu^{2+}$ is likely; if yellow-brown, $Fe^{3+}$ is likely.
Step 2: Add dilute NaOH dropwise: A blue gelatinous precipitate confirms $Cu(OH)_2$ (Cu present). A rust-brown precipitate confirms $Fe(OH)_3$ (Fe present). A white precipitate that dissolves in excess NaOH suggests $Pb(OH)_2$ , which is amphoteric.
Step 3: Confirm $Pb^{2+}$: To a fresh portion, add dilute KI. A bright yellow precipitate of $PbI_2$ confirms lead:

Pb^{2+} + 2I^- \rightarrow PbI_{2(s)}

Step 4: Test for chloride. To a fresh portion (acidified with dilute $HNO_3$ to suppress carbonate interference), add silver nitrate. A white precipitate that dissolves in ammonia confirms $Cl^-$ .

Step 5: Test for sulfate. To a fresh portion (acidified with dilute HCl to remove carbonate), add barium chloride. A white precipitate of $BaSO_4$ that does not dissolve in acid confirms $SO_4^{2-}$ .

Always use fresh portions for each test; never add reagents sequentially to one sample because precipitates and complexes interfere with later tests.

Markers reward (1) a logical sequence (inspection then NaOH then specific tests), (2) at least one named precipitate per ion, (3) the acidification step to remove carbonate interference, (4) the principle of using fresh portions.