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wise otter.Chemistry
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Data and Experimental Skills

Topic 7 of 8

Reason from qualitative tests

Keep the observation separate from the inferred identity.

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

Reason from an unknown test without guessing

A valid inference needs the right reagent, a clear observation and an appropriate comparison.

Use clean apparatus and a fresh small portion for each test. Add reagent gradually with mixing; observe a few drops, excess reagent and warming separately when instructed. Write the actual colour and physical change. A blue solution and a pale-blue precipitate are different observations. Name a gas only after the appropriate confirmatory test.

Worked example

Use two observations together

An unknown gives a white precipitate with NaOH that dissolves in excess. A fresh portion gives a white precipitate with ammonia that remains in excess.

  1. The NaOH result fits more than one possible cation; it is not a unique identification.
  2. Compare the ammonia result with the reference table: zinc hydroxide dissolves in excess ammonia, whereas aluminium hydroxide remains.
  3. Together, the observations support Al3+ among these candidates. Do not mix the reagents in one test portion and call the outcomes independent.
Answer

Use the pair of tests to distinguish candidates, with the observation and inferred ion recorded separately.

The supplied qualitative-analysis notes are a reference, not a reason to skip observation. When a reaction involves an ion outside the specified list, a question may ask only for a general deduction such as gas evolution, precipitation or redox behaviour. Do not assign an unfamiliar colour to an invented identity.

For a simple organic test, record the reagent and what changes. Bromine water changing from orange to colourless is consistent with an alkene under the relevant conditions, but decolourisation alone is not a universal proof of C=C because other substances can also react. Use the stated candidate structures and any other evidence to narrow the conclusion.

For a halide or sulfate test, avoid introducing the target ion through the acid used for acidification. In a nitrate test, ammonium can also produce ammonia on warming with alkali; account for that possibility before attributing the gas to nitrate reduction. Treat a missing visible precipitate cautiously if concentration could be too low.

Check your understandingA solid disappears after excess reagent is added, leaving a clear coloured liquid. How should this be recorded?Think it through, then reveal the answer
State the original precipitate colour, that it dissolves in excess reagent, and the final solution colour. "The colour disappears" is inaccurate if the remaining solution is still coloured.