A level Chemistry · Edexcel 9CH0 · Topic 4C
Tests for ions
Key points
- Flame test: dip a clean nichrome wire (cleaned with conc. HCl) into the sample and hold in a blue flame; note the colour.
- Ammonium ions: warm with NaOH(aq); ammonia gas turns damp red litmus blue.
- Carbonate/hydrogencarbonate: add dilute acid; gas turns limewater milky (CO₂).
- Sulfate: acidify with dilute HCl, then add BaCl₂(aq); a white precipitate of BaSO₄ forms. The acid removes carbonate first.
- Halides: acidify with dilute HNO₃, then add AgNO₃(aq): AgCl white, AgBr cream, AgI yellow.
- Confirm halides with ammonia: AgCl dissolves in dilute NH₃, AgBr only in concentrated NH₃, AgI is insoluble in both.
- Hydroxide ions: test with an indicator; solutions with pH above 7 are alkaline.
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Qualitative analysis means identifying the ions in an unknown substance from observations. You need to know each test, the positive result and why the conditions are chosen, because these are tested in the practical (CP7) and in written…
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