Chemical tests to identify common ions
Chemical analysis • Identification of ions
Flashcards
Test your knowledge with interactive flashcards
Key concepts
What you'll likely be quizzed about
Ions and simple definitions
Ions represent charged particles formed when atoms gain or lose electrons; positive ions are cations and negative ions are anions. Single ionic compounds contain one cation and one anion, so identification focuses on two components and their observable chemical behaviour. Exact identification depends on characteristic reactions rather than appearance alone.
Flame tests for metal cations
Heating a sample in a flame excites electrons in metal ions; the subsequent return to lower energy levels emits light with characteristic colours. Lithium produces a crimson flame; sodium produces an intense yellow flame; potassium produces a lilac flame; calcium produces an orange-red flame; copper(II) produces a blue-green flame. Colour intensity and contamination affect reliability, so flame tests act as a rapid preliminary identification.
Sodium hydroxide precipitation tests for cations
Adding aqueous sodium hydroxide to a solution causes metal ions to form hydroxide precipitates when insoluble. Copper(II) forms a blue precipitate, iron(II) forms a pale green precipitate, iron(III) forms a brown precipitate, and calcium and magnesium form white precipitates. Aluminium forms a white precipitate that dissolves in excess sodium hydroxide; zinc does the same. Solubility in excess reagent distinguishes some ions.
Tests for anions: carbonates, sulfates and halides
Carbonate ions react with dilute acid to produce carbon dioxide gas, which turns limewater cloudy; the cause is acid-induced decomposition of carbonate into CO2. Sulfate ions produce a white barium sulfate precipitate when barium chloride is added to an acidified sample because barium sulfate is insoluble. Halide ions give silver halide precipitates with silver nitrate: chloride produces white, bromide produces cream, and iodide produces yellow precipitates; acidification with nitric acid removes carbonate interference before the test.
Distinguishing halides with aqueous ammonia
Solubility of silver halides in aqueous ammonia provides confirmation: silver chloride dissolves in dilute ammonia, silver bromide dissolves only in concentrated ammonia, and silver iodide remains insoluble. The cause is complex formation between silver ions and ammonia, which increases solubility for some silver halides and allows sequential testing to identify the specific halide.
Tests for ammonium and nitrate ions
Ammonium ions produce ammonia gas when warmed with sodium hydroxide; ammonia gas turns moist red litmus paper blue. Nitrate ions produce ammonia when reduced by aluminium in strongly alkaline conditions (warm NaOH with aluminium), so the same ammonia test indicates nitrate after reduction. Gas evolution and litmus change provide clear qualitative signals for these ions.
Order of tests and avoiding false positives
Testing order prevents interference: acidify before sulfate and halide tests to remove carbonate; perform carbonate test first with dilute acid to observe CO2 before acidification for other tests. Contamination and overlapping reactions cause false positives, so confirmation by two different tests (for example, flame test plus a precipitation test) increases confidence in identification.
Limitations and practical factors
Low concentration, mixed samples and coloured ions reduce visibility of expected results; some precipitates form slowly or redissolve in excess reagent. Instrumental methods provide greater sensitivity, but simple chemical tests remain useful for clear single-compound samples when performed carefully and with controls.
Key notes
Important points to keep in mind