Testing for negative ions

The chemical tests for halides, carbonates, sulfates and nitrates

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Test for carbonate

  • add dilute HCl to the substance
  • Carbonates fizz because they give off carbon dioxide
  • Carry out the limewater test
  • If the lime water goes milky then carbonates are present
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  • dissolve sulfate in distilled water
  • Acidify the solution with dilute HCl
  • Put a small amount of solution in a test tube
  • Add barium chloride drop by drop
  • Look for a dense white precipitate - this shows sulfate ions are present
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Halide (chloride, bromide, iodide)

  • Dissolve in distilled water
  • Put 2cm depth of halide solution in a test tube
  • Acidify the solution with dilute nitric acid
  • Add silver nitrate solution drop by drop
  • If a precipitate forms, look at the colour to identify the halide:
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  • Dissolve in distilled water
  • Put 2cm of solution in a boiling tube
  • Add a few drops of sodium hydroxide
  • Warm gently to boiling
  • If it smells like ammonia and turns damp red litmus blue, there is an ammonia salt - test won't work properly
  • IF no ammonia - add a pinch of aluminium powder and boil gently
  • A positive ammonia test now shows that a nitrate is present
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