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K324 / 2027
Qualitative Analysis

Lesson 4 of 4

Turn observations into a justified conclusion

Use independent evidence and distinguish uncertainty from absence.

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

  • I can identify aqueous cations.Syllabus

    Syllabus K324, 6(a). Identify aqueous cations

  • I can identify anions.Syllabus

    Syllabus K324, 6(b). Identify anions

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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