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K324 / 2027
Qualitative Analysis

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Qualitative Analysis

Identify ions and gases using observations that distinguish them.

O-Level 6092 (2026) / SEC G3 K324 (2027)

01

Distinguish aqueous cations

Observe a few drops first, then add the reagent in excess.

Make a guess. You are not marked.

Aqueous sodium hydroxide is added to a solution of iron(II) ions and a green precipitate forms. The tube is left open to the air. What happens to the precipitate?

  1. It turns red-brown on standing, as air oxidises it
  2. It stays green, as the precipitate is already formed
  3. It dissolves to a colourless solution over time
Show the answer

It turns red-brown on standing, as air oxidises it

Oxygen in the air oxidises iron(II) hydroxide to iron(III) hydroxide, which is red-brown. Mark schemes reward this observation.

Use a fresh small portion of the unknown for each reagent. Add aqueous sodium hydroxide or aqueous ammonia a little at a time, mix, and record any precipitate. Then add excess reagent and record whether that precipitate dissolves. A precipitate is an insoluble solid, not simply a coloured solution.

Cation observations
CationAqueous NaOHAqueous ammonia
Al3+White precipitate; dissolves in excess to a colourless solutionWhite precipitate; insoluble in excess
Ca2+White precipitate; insoluble in excessNo precipitate
Cu2+Light blue precipitate; insoluble in excessLight blue precipitate; dissolves in excess to a dark blue solution
Fe2+Green precipitate; insoluble in excess; turns red-brown on standingGreen precipitate; insoluble in excess; turns red-brown on standing
Fe3+Red-brown precipitate; insoluble in excessRed-brown precipitate; insoluble in excess
Zn2+White precipitate; dissolves in excess to a colourless solutionWhite precipitate; dissolves in excess to a colourless solution
NH4+On warming, ammonia evolves; test with damp red litmusNot a useful ammonium test: the reagent already contains ammonia

Most precipitates in this table are metal hydroxides. For example, Cu2+(aq) + 2OH-(aq) -> Cu(OH)2(s). The copper ion can give a clear blue solution before reaction and a cloudy light-blue precipitate afterwards; describe both colour and physical change. Complex-ion formulae for reactions with excess ammonia are not required.

Worked exampleDistinguish aluminium from zinc

An unknown forms a white precipitate soluble in excess NaOH. What next?

Try it first. Then show the worked stepsHide the worked steps
  1. This result fits both Al3+ and Zn2+; it is not enough to identify either.
  2. Test a fresh portion with aqueous ammonia, then excess.
  3. A white precipitate remaining in excess supports aluminium; dissolution to a colourless solution supports zinc.
Answer

Use a second discriminating test, rather than repeating the same ambiguous observation.

Check your understanding

With aqueous ammonia, an unknown gives a light-blue precipitate that dissolves in excess to a dark-blue solution. Which cation is likely present?

  1. Zn2+, since its precipitate also dissolves in excess aqueous ammonia
  2. Fe2+, whose green precipitate looks blue at first in excess ammonia
  3. Cu2+, which gives both the blue precipitate and the dark-blue solution
Show the answer

Cu2+, which gives both the blue precipitate and the dark-blue solution

Copper(II) hydroxide is a light-blue precipitate, and it dissolves in excess ammonia to give a dark-blue solution. Record both stages as evidence.

Exam-style question

Aqueous ammonia is added a little at a time, until in excess, to a solution containing copper(II) ions. Describe what you would observe. [2]

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Sort these

Aqueous sodium hydroxide is added to each cation until it is in excess. Sort the cations by what you see.

Show the answer

Precipitate dissolves in excess: Al3+ / Zn2+

Precipitate stays in excess: Ca2+ / Fe3+ / Cu2+ / Fe2+

No precipitate; ammonia given off on warming: NH4+

02

Identify anions with the correct reagents

Acidification matters because contaminants can imitate the result.

Anion tests
AnionProcedurePositive observation
CO32-Add dilute acid; pass evolved gas into limewaterEffervescence; gas forms a white precipitate in limewater
Cl-Acidify solution with dilute nitric acid, then add aqueous silver nitrateWhite precipitate of AgCl
SO42-Acidify solution with dilute nitric acid, then add aqueous barium nitrateWhite precipitate of BaSO4
NO3-Add aqueous NaOH and aluminium foil, then warm carefully; test the gasAmmonia turns damp red litmus blue
Pure only

Pure also requires iodide: acidify with dilute nitric acid and add aqueous silver nitrate. A yellow precipitate of AgI supports I-. Ag+(aq) + I-(aq) -> AgI(s). Compare its colour with white AgCl rather than describing every pale precipitate as white.

Nitric acid removes interfering carbonate before the silver or barium test. Do not use hydrochloric acid to acidify for chloride: it introduces the ion being tested. Likewise, sulfuric acid would introduce sulfate. Record the original sample result using clean apparatus and separate portions; do not carry reagents from one test into another.

For nitrate, aluminium in alkaline solution reduces nitrate to ammonia. Ammonium ions also release ammonia with warm NaOH, even without aluminium. If ammonium could be present, check that separately and account for it before attributing ammonia to nitrate. Formulae of complex products are not needed for this identification.

Check your understanding

A student acidifies an unknown with hydrochloric acid before adding silver nitrate. Why is a white precipitate inconclusive?

  1. The acid reacts with silver nitrate to make silver gas, which clouds the solution.
  2. The acid adds chloride ions, so AgCl could form even if the sample had none.
  3. The acid dissolves any precipitate, so a white solid must come from something else.
Show the answer

The acid adds chloride ions, so AgCl could form even if the sample had none.

Use dilute nitric acid for this test. Hydrochloric acid supplies Cl- itself, so the white precipitate cannot show that chloride was in the sample.

03

Identify the gas, not just the bubbles

A named gas needs a specific test and observation.

Six gas tests
GasTestPositive observation
Hydrogen, H2Apply a lighted splint to a small collected sampleA pop
Oxygen, O2Insert a glowing splintSplint relights
Carbon dioxide, CO2Bubble through limewaterWhite precipitate; prolonged excess CO2 can dissolve it
Ammonia, NH3Hold damp red litmus in the gasTurns blue
Chlorine, Cl2Expose damp litmus paper to the gasBleaches; damp blue litmus may first redden
Sulfur dioxide, SO2Use acidified potassium manganate(VII) solutionPurple solution becomes colourless

Litmus must be damp so the gas can dissolve and show its acidic or alkaline effect. Do not identify a gas by smell. Write "effervescence" for bubbles, then describe the confirmatory test. Carbon dioxide extinguishing a flame is less diagnostic than limewater; several gases fail to support combustion.

The sulfur dioxide test is required theory, but candidates are not required to carry out tests involving sulfur dioxide gas in the practical examination. Its colour change reflects reducing behaviour; other reducing substances can also decolourise acidified manganate(VII), so use the experimental context.

Check your understanding

A glowing splint goes out in a gas. Does that identify hydrogen?

  1. No. Hydrogen is tested with a lighted splint, which gives a pop.
  2. Yes. Hydrogen puts out a glowing splint because it is flammable.
  3. No. It shows the gas is oxygen, which puts out a glowing splint.
Show the answer

No. Hydrogen is tested with a lighted splint, which gives a pop.

Many gases put out a glowing splint, such as CO2 and N2, so this does not identify the gas. The test for hydrogen is a squeaky pop with a lighted splint.

04

Turn observations into a justified conclusion

Use independent evidence and distinguish uncertainty from absence.

A disciplined unknown investigation
  1. Observe before adding reagents

    Record colour, state and whether the sample is already cloudy.

  2. Test fresh portions

    Keep cation, anion and gas tests independent.

  3. Record sequence

    A few drops, excess reagent and warming may give different results.

  4. Infer only what the evidence supports

    Use a second test when several ions fit. Negative results also eliminate possibilities.

Write observations the way mark schemes do
Write thisNot thisWhy
White precipitateWhite solid; cloudy; milky solutionA precipitate is the named observation for an insoluble solid forming in a solution.
Green precipitate, turns red-brown on standingGreen precipitate (only)Iron(II) hydroxide is oxidised by air; the colour change is a separate mark.
Precipitate dissolves in excess to give a colourless solutionPrecipitate disappears"Dissolves" names the change; give the colour of the final solution.
Effervescence; the gas gives a white precipitate with limewaterBubbles of carbon dioxideEffervescence is what you see. Carbon dioxide is the inference, proved by the limewater.
Damp red litmus paper turns blueAmmonia gas is given offReport the test result first, then name the gas.
Orange to colourlessGoes clear; turns colourlessGive the colour before and after. Clear is not a colour.
No visible change; no precipitateNothing happensSay what you looked for and did not see.

Worked exampleIdentify a dissolved salt

Fresh portions of a solution give a green precipitate with NaOH, insoluble in excess, and a white precipitate with acidified barium nitrate.

Try it first. Then show the worked stepsHide the worked steps
  1. The cation result supports Fe2+, not Fe3+ which gives red-brown hydroxide.
  2. The acidified barium test supports SO42-.
  3. Combining ion charges gives FeSO4.
Answer

Iron(II) sulfate is consistent with the evidence. State the observations separately from this inferred identity.

"No change" needs context: no precipitate with ammonia may distinguish calcium from zinc, but a very dilute sample could also make a precipitate hard to see. In an unfamiliar test, report what occurred and draw a general conclusion supported by the instructions; do not invent an ion name from an unfamiliar colour.

Check your understanding

Which of these is an observation rather than an inference?

  1. Silver chloride forms when silver nitrate is added.
  2. Chloride ions are present in the solution being tested.
  3. A white precipitate forms when silver nitrate is added.
Show the answer

A white precipitate forms when silver nitrate is added.

An observation is what you see. "Chloride is present" is an inference you draw from that observation, after a correct test with nitric acid and silver nitrate.

Check your understanding

Aqueous sodium hydroxide is added to iron(II) sulfate solution and the tube is left to stand. Which is the best recorded observation?

  1. Green precipitate, insoluble in excess, turning red-brown on standing
  2. A green solid forms in the tube, and then it slowly goes brown
  3. The solution turns green and cloudy, and then turns red-brown
  4. Iron(II) hydroxide forms, and later it is oxidised to iron(III)
Show the answer

Green precipitate, insoluble in excess, turning red-brown on standing

It names the precipitate and its colour, says what happens in excess, and gives the change on standing, each in the words mark schemes credit.

Exam-style question

A white solid P is tested as follows.
1. P + dilute hydrochloric acid → effervescence; gas Q gives a white precipitate with limewater. A colourless solution R forms.
2. R + aqueous sodium hydroxide, a little at a time → white precipitate S, which dissolves in excess to give a colourless solution.
3. R + aqueous ammonia, a little at a time → white precipitate, which dissolves in excess to give a colourless solution.
4. R + dilute nitric acid, then aqueous silver nitrate → white precipitate T.
(a) Identify P, Q, S and T. [4]
(b) Write the ionic equation, with state symbols, for the formation of S. [1]
(c) A student says test 4 shows that P contains chloride ions. Explain why this is wrong. [1] [6]

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Quick revision

Revisit the essentials, then return to an explanation when you need it.

For cations, record precipitate colour and behaviour in excess reagent. Use fresh portions. For chloride/sulfate tests acidify with nitric acid; do not introduce the ion being tested.

Pure only

Bubbles alone do not identify a gas. State the gas test and its positive observation. Iodide testing is an additional Pure requirement.

Check your understanding

Al3+ and Zn2+ both give white precipitates that dissolve in excess NaOH. Which reagent tells them apart?

  1. More NaOH, warmed: zinc's precipitate comes back, aluminium's stays dissolved.
  2. Aqueous ammonia: zinc's precipitate dissolves in excess, aluminium's does not.
  3. Silver nitrate, which gives a white precipitate with Al3+ but none with Zn2+.
Show the answer

Aqueous ammonia: zinc's precipitate dissolves in excess, aluminium's does not.

Use a fresh portion. Both give white precipitates with ammonia, but only zinc hydroxide dissolves in excess ammonia, giving a colourless solution.

Scope

Learning outcomes

6. Qualitative Analysis (6092 / K324). Use the outcome map to find the explanation for a particular syllabus requirement.

See the learning outcome map

  1. 6(a) Identify aqueous cations

    • NaOH and/or ammonia; precipitate formation and excess solubility
    • Al3+, NH4+, Ca2+, Cu2+, Fe2+, Fe3+, Zn2+
    • Ammonium warming and evolved gas
    • Complex-ion formulae not required

    Distinguish aqueous cationsTurn observations into a justified conclusion

  2. 6(b) Identify anions

    • Carbonate with dilute acid and limewater
    • Chloride and iodide with nitric acid and silver nitrate
    • Nitrate with aluminium/NaOH, warming and damp red litmus
    • Sulfate with nitric acid and barium nitrate

    Identify anions with the correct reagentsTurn observations into a justified conclusion

  3. 6(c) Identify gases

    • NH3 damp red litmus
    • CO2 limewater
    • Cl2 damp litmus
    • H2 lighted splint
    • O2 glowing splint
    • SO2 acidified manganate(VII)

    Identify the gas, not just the bubbles