3.1.4 - qualitative analysis

    Cards (19)

    • Anions are negative ions
    • Cations are positive ions
    • To test for carbonate ions:
      add strong hydrochloric acid
      collect the gas produced
      pass through limewater
    • An equation for the carbonate ion test
      CO3 + H+ ————> H2O + CO2
    • To test for sulfate ions:
      add dilute hydrochloric acid and barium chloride to the sample
    • if sulfate ions are present, you observe a white precipitate of barium sulfate
    • An equation for the sulfate ion test is:
      Ba + SO4 ———> BaSO4
    • To test for halide ions you use silver nitrate
    • You use silver nitrate for halide ions instead of HCl because it removes carbonate ions.
      Adding HCl would add Cl- ions giving it a false positive result
    • To test for a halide
      dissolve sample in water
      add aqueous silver nitrate
      record colour change
      if it is difficult to distinguish the colour add aqueous ammonia (dilute first then concentrated)
      note the solubility of precipitate
    • If chloride ions are present, a white precipitate is formed. And it is soluble in dilute aqueous ammonia.
    • The equation for the chloride test is:
      Ag + Cl ——> AgCl
    • If bromide ions are present, a cream precipitate is formed. It is only soluble in concentrated aqueous ammonia.
    • The equation for the bromide test is:
      Ag + Br ——> AgBr
    • The equation for the iodide test is:
      Ag + I ——> AgI
    • The order of testing for anions should be
      1. carbonate
      2. sulfate
      3. halide
      this is because barium ions form an insoluble precipitate of barium carbonate
      and silver ions form insoluble precipitate of silver sulfate
    • To test for ammonium ions you
      add sodium hydroxide to the sample and gently warm it
      test the gas produced with red litmus paper
    • If ammonium ions are present the red litmus paper turns blue and ammonia has a distinct pungent smell
    • The equation for the ammonium ions test is:
      NH4+ + OH- ——> NH3 + H2O