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

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 OpusReviewed by Better Tuition Academy10 min answerUpdated 2026-05-18

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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
IMATH_4	Carmine red
IMATH_5	Persistent yellow-orange
IMATH_6	Lilac (use cobalt-blue glass to block sodium)
IMATH_7	Brick red
IMATH_8	Crimson
IMATH_9	Apple green
IMATH_10	Blue-green
IMATH_11	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	IMATH_12	IMATH_13	IMATH_14	IMATH_15	IMATH_16	IMATH_17	IMATH_18 , IMATH_19
Dilute NaOH	Blue gel	Dirty green	Rust brown	White (redissolves in excess)	Brown IMATH_20	White	No ppt (Ca slow)
Dilute IMATH_21	Blue, then deep blue with excess (complex)	Green, darkens	Brown	White	Brown then dissolves in excess	White	No ppt
IMATH_22	Green-blue	Green-white	Brown	White	Pale yellow	White	White
IMATH_23	Brown	(Pale)	(Brown)	Yellow IMATH_24	Red IMATH_25	(No)	Yellow $BaCrO_4$ , faint IMATH_27

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
IMATH_30	IMATH_31 in dilute IMATH_32	White ppt of $AgCl$ , dissolves in dilute IMATH_34
IMATH_35	IMATH_36 in dilute IMATH_37	Cream ppt of $AgBr$ , partly dissolves in concentrated IMATH_39
IMATH_40	IMATH_41 in dilute IMATH_42	Yellow ppt of $AgI$ , insoluble in IMATH_44
IMATH_45	IMATH_46 in dilute IMATH_47	White ppt of $BaSO_4$ , insoluble in acid
IMATH_49	Dilute IMATH_50	Effervescence of $CO_2$ , turns limewater milky
IMATH_52	IMATH_53 in neutral solution	Yellow ppt of IMATH_54
IMATH_55	Universal indicator or pH	Blue/purple, pH > 10
IMATH_56	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.

In one sentence

Identify cations by colour, flame test and precipitation with $NaOH/NH_3/CO_3^{2-}$ , identify anions by acidified $AgNO_3$ (halides), acidified $BaCl_2$ (sulfate) or $HCl$ (carbonate), confirm with complexation ( $Ag^+$ with $NH_3$ , $Fe^{3+}$ with $SCN^-$ , $Cu^{2+}$ with excess $NH_3$ ), and always work on fresh portions with the right acid background.

Past exam questions, worked

Real questions from past NESA papers on this dot point, with our answer explainer.

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.

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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: $Pb^{2+}$, $Cu^{2+}$, $Fe^{3+}$, $Cl^-$, $SO_4^{2-}$.

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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.