Topic 7 of 8

Reason from qualitative tests

Keep the observation separate from the inferred identity.

A-Level 9476 (2026-2027)

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.

The NaOH result fits more than one possible cation; it is not a unique identification.
Compare the ammonia result with the reference table: zinc hydroxide dissolves in excess ammonia, whereas aluminium hydroxide remains.
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.

Use the H2 inorganic test tables

Compare both the initial change and behaviour in excess.

H2 cations: aqueous reagents, then excess
Ion	NaOH(aq)	NH₃(aq)
Al³⁺	White precipitate; dissolves in excess	White precipitate; remains in excess
NH₄⁺	Ammonia released on warming	Reagent contains ammonia; not a diagnostic test
Ba²⁺	No precipitate with pure reagents	No precipitate
Ca²⁺	White precipitate at sufficiently high calcium concentration	No precipitate
Cr³⁺	Grey-green precipitate; dissolves in excess to dark green solution	Grey-green precipitate; remains
Cu²⁺	Pale-blue precipitate; remains	Blue precipitate; dissolves in excess to dark-blue solution
Fe²⁺	Green precipitate, browns in air; remains	Green precipitate, browns in air; remains
Fe³⁺	Red-brown precipitate; remains	Red-brown precipitate; remains
Mg²⁺	White precipitate; remains	White precipitate; remains
Mn²⁺	Off-white precipitate, rapidly browns in air; remains	Off-white precipitate, rapidly browns in air; remains
Zn²⁺	White precipitate; dissolves in excess	White precipitate; dissolves in excess

H2 anions: interpret the specified test
Ion	Evidence	Qualification
CO₃^2-	Dilute acid releases CO₂; test with limewater	Effervescence alone is not an identification
Cl^-	White AgCl with acidified silver nitrate; dissolves in aqueous ammonia	Use nitric acid, which introduces no halide
Br^-	Pale-cream AgBr; less soluble in ammonia than AgCl	The supplied notes describe partial solubility; concentration matters
I^-	Yellow AgI; remains in ammonia	Compare colour and solubility together
NO₃^-	Ammonia on heating with alkali and aluminium	Account for any ammonium already present
SO₄^2-	White BaSO₄, insoluble in excess dilute strong acid	Acid removes carbonate interference
NO₂^-	With dilute acid, NO forms and becomes brown NO₂ in air; also gives ammonia with alkali/Al	Reference interpretation; excluded as a hands-on H2 test
SO₃^2-	Dilute acid releases SO₂; barium precipitate dissolves in dilute strong acid	Reference interpretation; sulfite/SO₂ practical tests are excluded

Gas evidence
Gas	Observation in the named test
NH₃	Damp red litmus turns blue
CO₂	Limewater forms a white precipitate; excess gas can dissolve it
Cl₂	Damp litmus is bleached
H₂	Small sample gives a pop with a lighted splint
O₂	Glowing splint relights
SO₂	Acidified manganate(VII) changes from purple to colourless; theory/reference only for this practical scope

Halogen colours in the supplied H2 reference
Halogen	Element	Aqueous solution	Hexane solution: reference only
Cl₂	Greenish-yellow gas	Pale yellow	Pale yellow
Br₂	Reddish-brown gas or liquid	Orange	Orange-red
I₂	Black solid; purple gas	Brown	Purple

A precipitate dissolving in excess may reflect amphoteric behaviour or complex formation; use the actual reagent and chemical context. A colour change on standing can be evidence of oxidation by air. Do not silently replace the immediate observation with the later one.

Check your understandingA precipitate is green immediately and brown after standing in air. Is it adequate to report only "brown precipitate"?Think it through, then reveal the answer

No. Preserve the sequence. The change can support initial iron(II) hydroxide followed by oxidation, whereas an immediately red-brown iron(III) precipitate is different evidence.