Qualitative Analysis: Tests for Ions

Qualitative analysis involves identifying which ions are present in an unknown sample using chemical tests. For Edexcel A-Level, you need to know flame tests for metal cations, precipitation tests with sodium hydroxide, tests for common anions, and tests for gases. This is one of the most practical and frequently examined topics.

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Flame Tests

Flame tests identify metal ions by the characteristic colour they produce when heated in a Bunsen flame. Electrons are promoted to higher energy levels by the heat, and as they fall back down, they emit light of specific wavelengths.

Procedure

1. Clean a nichrome wire loop by dipping it in concentrated hydrochloric acid and holding it in a blue Bunsen flame until no colour is produced
2. Dip the clean wire into the sample (or into concentrated HCl and then into the solid)
3. Hold the wire in the edge of the blue flame and observe the colour

Flame Colours

Metal Ion	Flame Colour	Characteristic Description
Li⁺	Crimson / red	Deep, rich red
Na⁺	Bright yellow / orange	Intense and persistent
K⁺	Lilac / purple	Pale and easily masked by Na
Ca²⁺	Brick red / orange-red	Distinct from Li (more orange)
Sr²⁺	Crimson / deep red	Can be confused with Li
Ba²⁺	Pale green / apple green	Distinctive
Cu²⁺	Blue-green / green-blue	Can be confused with Ba

Practical tips:

• Sodium contamination is very common (sodium is present on skin, glassware, and in many chemicals). A bright yellow sodium flame can mask other colours.
• Looking through blue cobalt glass filters out the yellow sodium colour, making it easier to see the lilac potassium flame.
• Li⁺ (crimson) and Sr²⁺ (crimson) can be confused. Use other tests (NaOH precipitation) to distinguish them if needed.
• Ba²⁺ (pale green) and Cu²⁺ (blue-green) can appear similar. Ba²⁺ compounds are typically white/colourless in solution; Cu²⁺ compounds are blue in solution.

flowchart TD
    A["Flame Test Observation"] --> B{"Colour?"}
    B -->|"Crimson/red"| C{"Other tests needed"}
    C -->|"White compound, no colour with NaOH"| D["Li⁺ or Sr²⁺"]
    C -->|"Coloured precipitate with NaOH"| E["Not Li/Sr – re-examine"]
    B -->|"Bright yellow"| F["Na⁺"]
    B -->|"Lilac (view through<br>cobalt glass)"| G["K⁺"]
    B -->|"Brick red/orange-red"| H["Ca²⁺"]
    B -->|"Pale green"| I["Ba²⁺"]
    B -->|"Blue-green"| J["Cu²⁺"]

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Testing for Cations with NaOH Solution

Adding sodium hydroxide solution to a solution containing transition metal cations produces characteristic coloured precipitates. These precipitates are insoluble metal hydroxides.

Results Table

Metal Ion	Precipitate Formula	Colour	With Excess NaOH
Fe²⁺	Fe(OH)₂	Green (darkens on standing)	No change — insoluble
Fe³⁺	Fe(OH)₃	Red-brown / rust	No change — insoluble
Cu²⁺	Cu(OH)₂	Blue	No change — insoluble
Mn²⁺	Mn(OH)₂	Pale brown / buff	No change — insoluble
Al³⁺	Al(OH)₃	White	Dissolves in excess (amphoteric)
Cr³⁺	Cr(OH)₃	Green	Dissolves in excess (amphoteric)

Key distinctions to learn:

Fe(OH)₂ (green) darkens to brown on standing as it is oxidised by air to Fe(OH)₃:

4Fe(OH)₂(s) + O₂(g) + 2H₂O(l) → 4Fe(OH)₃(s)

Amphoteric hydroxides: Al(OH)₃ and Cr(OH)₃ dissolve in excess NaOH because they are amphoteric:

Al(OH)₃(s) + NaOH(aq) → NaAlO₂(aq) + 2H₂O(l) Cr(OH)₃(s) + NaOH(aq) → NaCrO₂(aq) + 2H₂O(l)

This dissolving in excess NaOH distinguishes Al³⁺ and Cr³⁺ from the other metal ions.

Distinguishing Fe²⁺ from Cr³⁺ (Both Give Green Precipitates)

This is a classic exam trap. Both give green precipitates with NaOH, but:

• Fe(OH)₂: Does NOT dissolve in excess NaOH; darkens to brown on standing
• Cr(OH)₃: DOES dissolve in excess NaOH (amphoteric); does NOT darken

Testing with Ammonia (NH₃) Solution

Some metal hydroxides dissolve in excess ammonia due to complex ion formation:

Metal Ion	With small amount NH₃	With excess NH₃
Cu²⁺	Blue precipitate Cu(OH)₂	Dissolves → deep blue [Cu(NH₃)₄(H₂O)₂]²⁺
Co²⁺	Blue-green precipitate	Dissolves → straw/yellow-brown [Co(NH₃)₆]²⁺
Ni²⁺	Green precipitate	Dissolves → blue-violet [Ni(NH₃)₆]²⁺
Fe²⁺	Green precipitate	Does NOT dissolve
Fe³⁺	Red-brown precipitate	Does NOT dissolve

The Cu²⁺ test is particularly important: pale blue solution → blue precipitate → deep blue solution is a classic sequence.

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Testing for Anions

Carbonate Ion (CO₃²⁻)

Add dilute hydrochloric acid. If carbonate is present, the acid reacts to produce carbon dioxide:

CO₃²⁻(s/aq) + 2HCl(aq) → CO₂(g) + H₂O(l) + 2Cl⁻(aq)

Test the gas with limewater — it turns milky (white precipitate of CaCO₃). Extended bubbling clears it again as soluble Ca(HCO₃)₂ forms.

Sulfate Ion (SO₄²⁻)

Add dilute hydrochloric acid (to remove carbonates) and then barium chloride solution:

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) — white precipitate

The white precipitate of barium sulfate is insoluble in acid, confirming sulfate ions.

Why HCl and not HNO₃? For the sulfate test, HCl is used because it removes interfering carbonate ions (which would form BaCO₃, also a white precipitate). HCl does not introduce any ions that would form precipitates with Ba²⁺. For the halide test, HNO₃ is used because HCl would introduce Cl⁻ ions, giving a false positive.

Halide Ions (Cl⁻, Br⁻, I⁻)

Add dilute nitric acid followed by silver nitrate solution:

• Cl⁻: white precipitate (AgCl) — dissolves in dilute NH₃
• Br⁻: cream precipitate (AgBr) — dissolves in concentrated NH₃ only
• I⁻: yellow precipitate (AgI) — does not dissolve in either

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Gas Tests

Gas	Test	Positive Result
Chlorine (Cl₂)	Damp litmus paper	Bleaches (turns white). Initially turns red then white
Ammonia (NH₃)	Damp red litmus paper	Turns blue (only common alkaline gas)
Carbon dioxide (CO₂)	Bubble through limewater	Turns milky (CaCO₃ precipitate)
Hydrogen (H₂)	Burning splint	Burns with a squeaky pop
Oxygen (O₂)	Glowing splint	Relights the glowing splint
Sulfur dioxide (SO₂)	Acidified K₂Cr₂O₇ paper	Turns from orange to green
Hydrogen sulfide (H₂S)	Lead ethanoate paper	Turns black (PbS formed)
Nitrogen dioxide (NO₂)	Observation	Brown gas, sharp/pungent smell

Additional Gas Test Details

Chlorine bleaches litmus because HClO (formed when Cl₂ dissolves in water on the damp paper) is a strong oxidising agent that destroys the dye.

Ammonia is the only common laboratory gas that is alkaline. It can also be confirmed by holding a glass rod dipped in concentrated HCl near the gas — dense white fumes of NH₄Cl form: NH₃(g) + HCl(g) → NH₄Cl(s).

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Complete Qualitative Analysis Decision Tree

flowchart TD
    A["Unknown compound"] --> B{"Flame test"}
    B -->|"Distinctive colour"| C["Identify cation<br>(Li, Na, K, Ca, Sr, Ba, Cu)"]
    B -->|"No colour / inconclusive"| D["Try NaOH test"]
    A --> E{"Add NaOH"}
    E -->|"White ppt, dissolves excess"| F["Al³⁺"]
    E -->|"Green ppt, dissolves excess"| G["Cr³⁺"]
    E -->|"Green ppt, no dissolve, darkens"| H["Fe²⁺"]
    E -->|"Red-brown ppt"| I["Fe³⁺"]
    E -->|"Blue ppt"| J["Cu²⁺"]
    E -->|"Pale brown ppt"| K["Mn²⁺"]
    A --> L{"Anion tests"}
    L -->|"Add HCl, gas turns limewater milky"| M["CO₃²⁻"]
    L -->|"Add HCl then BaCl₂, white ppt"| N["SO₄²⁻"]
    L -->|"Add HNO₃ then AgNO₃"| O{"Precipitate colour?"}
    O -->|"White, dissolves in dil NH₃"| P["Cl⁻"]
    O -->|"Cream, dissolves in conc NH₃"| Q["Br⁻"]
    O -->|"Yellow, insoluble in NH₃"| R["I⁻"]

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Worked Example: Identifying an Unknown Salt

A student has an unknown green solid. They perform the following tests:

1. Flame test: no distinctive colour
2. Dissolve in water: green solution
3. Add NaOH: green precipitate forms; does NOT dissolve in excess NaOH; on standing, the precipitate darkens to brown
4. Add dilute HNO₃ then AgNO₃: white precipitate that dissolves in dilute NH₃

Analysis:

• No flame colour + green precipitate with NaOH → transition metal (Fe²⁺ or Cr³⁺)
• Precipitate does NOT dissolve in excess NaOH → rules out Cr³⁺ (which is amphoteric)
• Precipitate darkens to brown → confirms Fe²⁺ (oxidation to Fe(OH)₃)
• White precipitate with AgNO₃, dissolves in dilute NH₃ → Cl⁻
• Answer: Iron(II) chloride, FeCl₂

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Common Exam Pitfalls

1. Fe²⁺ vs Cr³⁺: Both give green precipitates with NaOH. Use excess NaOH (Cr dissolves, Fe does not) and observe over time (Fe darkens)
2. Ba²⁺ vs Cu²⁺ in flame tests: Both can appear greenish. Check the solution colour — Cu²⁺ is blue, Ba²⁺ is colourless
3. Using the wrong acid: HNO₃ for halide tests (not HCl!), HCl for sulfate tests (not HNO₃!)
4. Forgetting to acidify: Both the sulfate test and halide test require acidification first to remove interfering ions
5. Confusing precipitate colours: AgCl is white, AgBr is cream (not white), AgI is yellow. Small differences matter

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Summary

Qualitative analysis uses flame tests, NaOH precipitation tests, anion tests (CO₃²⁻, SO₄²⁻, halides), and gas tests to identify ions. The key is knowing the specific colours, precipitates, and procedures, and recording observations accurately. Use the decision tree approach: identify the cation first (flame test or NaOH), then identify the anion (specific reagent tests).

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A-Level Deep Dive: Qualitative Analysis Tests

Spec mapping

Edexcel 9CH0 specification Topics 4 and 15 combined cover the systematic identification of inorganic ions through qualitative analysis: tests for cations (Group 1/2 by flame test, transition metals via NH₃ and NaOH precipitation patterns, NH₄⁺ via warming with NaOH), anions (CO₃²⁻ by dilute acid + limewater, SO₄²⁻ by acidified BaCl₂, halides by AgNO₃ and ammonia, NO₃⁻ via Devarda's alloy), and the integration of multiple tests into a logical identification sequence (refer to the official specification document for exact wording). Examined heavily in Paper 3 (9CH0/03) and synoptically in Paper 1 (9CH0/01). Synoptic links: Topic 8 (Redox), Topic 11 (Acid-base equilibria), Topic 15 (Complex ions) — every CP16 procedure draws on multiple Topic strands.

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Worked example with full mark scheme

Question (8 marks): A student is given an aqueous solution containing one cation and one anion from the list: NH₄⁺, Na⁺, Mg²⁺, Cu²⁺, Cl⁻, Br⁻, NO₃⁻, SO₄²⁻.

The student observes:

• Flame test: no characteristic colour.
• Add NaOH(aq), warm: pungent gas turns moist red litmus blue.
• Acidify another portion with dilute HNO₃, add AgNO₃: cream precipitate, soluble in concentrated NH₃.

(a) Identify the cation and anion, justifying each step. (6)

(b) Suggest a single confirmatory test that would distinguish between Mg²⁺ and NH₄⁺ if the flame test had been ambiguous. (2)

Solution with mark scheme:

(a) Cation analysis.

Flame test negative rules out Group 1 (Na yellow, K lilac) and Group 2 (Mg none, Ca brick-red, Sr crimson, Ba apple-green) except Mg²⁺ which gives no flame colour. Cu²⁺ would also fail to produce a strong flame colour (sometimes faint blue-green) so the test is not fully discriminating.

M1 — flame test interpreted; Mg²⁺ or NH₄⁺ candidates.

NaOH + warm: pungent gas turning red litmus blue indicates NH₃ evolution. Reaction: NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l).

M1 — NH₃ identification + balanced equation.

A1 — cation = NH₄⁺; reasoning explicit.

Anion analysis.

Acidify with dilute HNO₃ (removes carbonate, prevents Ag₂CO₃ false positive). AgNO₃ gives cream precipitate → AgBr. Soluble in conc. NH₃ confirms (AgCl soluble in dilute NH₃; AgBr only in conc.; AgI insoluble).

M1 — cream precipitate identified as AgBr.

M1 — solubility in NH₃ used to discriminate from AgCl/AgI.

A1 — anion = Br⁻.

(b) Distinguishing Mg²⁺ from NH₄⁺ when flame test ambiguous:

Add NaOH(aq) and warm:

• Mg²⁺: white precipitate Mg(OH)₂ forms (insoluble in excess NaOH).
• NH₄⁺: no precipitate, but pungent NH₃ gas evolves on warming (turns red litmus blue).

B1 — distinguishing observation noted.

B1 — equation: NH₄⁺ + OH⁻ → NH₃ + H₂O or Mg²⁺ + 2OH⁻ → Mg(OH)₂.

Total: 8 marks (M4 A2 B2).

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Specimen question modelled on the Edexcel 9CH0 Paper 3 format

Question (6 marks): An unknown white solid X is tested as follows:

Test	Observation
Heat the solid	Gas evolved that turned limewater milky
Dissolve a fresh sample in water; add dilute HNO₃, then AgNO₃	White precipitate, soluble in dilute NH₃
Flame test on solid	Brick-red flame

(a) Identify X. (2)

(b) Write balanced equations for the three observations and explain the logic for each step. (4)

Mark scheme decomposition by AO: