Confirmatory Anion Tests and Resolving Ambiguous Results
Several classic anion confirmatory tests can look similar at first glance, and telling them apart reliably means reading the FULL set of clues together, not stopping at the first positive result — a single test in isolation is often ambiguous.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Reading the AgNO3/ammonia test carefully distinguishes the three halides
Silver halides dissolve in aqueous ammonia to different degrees depending on the halide: AgCl dissolves readily and completely even in dilute ammonia; AgBr dissolves only partially, and only in concentrated ammonia, described as soluble with difficulty; AgI does not dissolve in ammonia at all. A precipitate described as light yellow and NOT completely dissolving in ammonia therefore specifically points to bromide, not chloride or iodide. Combined with a positive chromyl chloride test (specific to chloride) and a violet-to-orange colour change in an organic layer on adding excess concentrated HNO3 (the classic iodide confirmatory test — iodide is oxidised to violet I2, then further oxidised by excess acid), a full picture of chloride, bromide AND iodide together can be built up, purely from these three independent, halide-specific tests.
The brown ring test alone cannot distinguish nitrate from nitrite
Both nitrate and nitrite give a positive brown ring test (the FeSO4/conc. H2SO4 test), since both can form the brown [Fe(NO)(H2O)5]2+ complex under the test conditions — a positive brown ring result on its own is therefore genuinely ambiguous between the two. The confirmatory step that resolves the ambiguity is treating the mixture with copper turnings and concentrated H2SO4 on heating: genuine nitrate gives an INTENSIFIED brown NO2 fume under these more forcing conditions, while nitrite does not intensify the same way. So a brown ring test that is positive, but does NOT intensify with copper turnings and concentrated H2SO4 on heating, points specifically to NITRITE being present, not nitrate — and in a mixture where no bromide or iodide is otherwise indicated (which could separately cause a brown-ring-like colour from their own vapours), nitrite is the correct read of this exact clue combination. Always pair the brown ring test with this copper-turnings confirmatory step before concluding nitrate is present.
Which anions give a barium precipitate that redissolves in concentrated HCl
BaCl2 solution gives a white precipitate with sulfite (BaSO3) and with carbonate (BaCO3), and both of these precipitates DISSOLVE in concentrated hydrochloric acid, releasing SO2 or CO2 gas respectively as the acid protonates and decomposes the precipitated salt. Sulfate is the classic exception: BaSO4 is also a white precipitate with BaCl2, but it does NOT dissolve in concentrated HCl (its lattice energy is too high relative to what dilute acid protonation can overcome) — so testing solubility in concentrated HCl is exactly how sulfite/carbonate are distinguished from sulfate in a qualitative scheme. Nitrate does not precipitate with BaCl2 at all (Ba(NO3)2 is soluble), so it is never a candidate for this particular test.
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- ✓AgCl dissolves readily in ammonia; AgBr dissolves only partially/with difficulty in concentrated ammonia; AgI does not dissolve at all — use this gradation to identify the halide
- ✓The brown ring test alone cannot distinguish nitrate from nitrite — both give a positive result
- ✓Copper turnings + concentrated H2SO4 + heat intensifies the brown fume specifically for genuine nitrate; no intensification points to nitrite instead
- ✓BaSO3 and BaCO3 (from sulfite and carbonate) dissolve in concentrated HCl; BaSO4 (from sulfate) does not — this is the standard test to tell them apart
- ✓Ba(NO3)2 is soluble, so nitrate never gives a precipitate with BaCl2 in the first place
Do not conclude nitrate is present from a positive brown ring test alone. Nitrite gives the same positive result — only the copper-turnings-plus-concentrated-H2SO4 intensification step (or its absence) tells you which one is genuinely there.
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