Testing for ions and gases

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  • Created by: emews
  • Created on: 29-11-17 17:56
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  • Testing for ions and gases
    • It is possible to use a flame test to detect the presence of an alkali metal ion.
      • The compound is put through a blue Bunsen flame. The colour produced indicates which alkali metal ion is present in the compound.
    • Ammonium ions can be identified in a solution by adding dilute sodium hydroxide solution and gently heating.
      • If ammonium ions are present, they will be converted to ammonia gas.
        • It also turns damp red litmus paper or damp universal indicator paper blue.
          • It also turns damp red litmus paper or damp universal indicator paper blue.
    • To tell whether an unknown substance contains iron(II) nitrate or iron(III) nitrate, add a few drops of sodium hydroxide solution:
      • if you get a green precipitate, the unknown substance is iron(II) nitrate
      • if you get an orange-brown precipitate, the unknown substance is iron(III) nitrate
      • Note that it is the metal that determines the colour.
    • An acid, such as dilute hydrochloric acid, is added to the test compound for carbonate ions.
      • Carbon dioxide gas bubbles if carbonate ions are present.
    • Halide ions in solutions are detected using silver nitrate solutions.
      • The test solution is acidified using a few drops of dilute nitric acid, and then a few drops of silver nitrate solution are added.
      • Different coloured silver halide precipitates form, depending on the halide ions present:
        • chloride ions give a white precipitate of silver chloride
        • bromide ions give a cream precipitate of silver bromide
        • iodide ions give a yellow precipitate of silver iodide
    • Hydrogen, oxygen, carbon dioxide, ammonia and chlorine can be identified using different tests.
      • A lighted wooden splint makes a popping sound in a test tube of hydrogen.
      • A glowing wooden splint relights in a test tube of oxygen.
      • Carbon dioxide turns limewater milky.
      • It makes damp red litmus paper turn blue.
      • It also makes damp blue litmus paper turn red, and then bleaches it white.

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