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Qualitative Analysis

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Qualitative Analysis

Identify ions and gases using observations that distinguish them.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

01

Distinguish aqueous cations

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

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 excessGreen precipitate; insoluble in excess
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 example

Distinguish aluminium from zinc

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

  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 understandingAn ammonia test changes from a light-blue precipitate to a dark-blue solution in excess. What is the likely cation?Think it through, then reveal the answer
Cu2+. Include both stages: the final dark-blue solution alone omits useful evidence about the initial precipitate.
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

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 understandingA student acidifies an unknown with hydrochloric acid before adding silver nitrate. Why is a white precipitate inconclusive?Think it through, then reveal the answer
Hydrochloric acid supplies chloride ions, so AgCl could form even if the original sample contained none. Use dilute nitric acid for this test.
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 understandingA glowing splint is extinguished. Does that identify hydrogen?Think it through, then reveal the answer
No. Hydrogen is tested with a lighted splint and gives a pop. An extinguished glowing splint only shows that the gas did not relight it; it does not uniquely identify the gas.
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.

Worked example

Identify 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.

  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 understandingWhich is an observation: "chloride is present" or "a white precipitate forms"?Think it through, then reveal the answer
A white precipitate is the observation. Chloride present is an inference that requires the correct silver-nitrate test and suitable acidification.

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.

Check your understandingWhich reagent distinguishes Al3+ from Zn2+ after both dissolve in excess NaOH?Think it through, then reveal the answer
Aqueous ammonia on a fresh portion: aluminium hydroxide stays insoluble in excess; zinc hydroxide dissolves.

Scope and references

Learning outcomes and sources

6. Qualitative Analysis (5086 / 5088 / K326 / K328). 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 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