Tests for negative ions

• what is the test for carbonate atoms?

  - add hydrochloric acid

  - effervescence is produced due to hydrogen

  - bubble through limewater

  - if limewater turns milky/cloudy, is present
• what is the test for negative sulphate ions?

  - add drops of dilute hydrochloric acid

  - add drops of barium chloride (BaCl2)

  - forms white precipitate if present
• test for ammonium negative ions

  -add sodium hydroxide

  - test gas using litmus paper

  - if litmus paper turns red it is not present

  - if litmus paper turns blue it is present
• test for nitrate negative ions

  - add sodium hydroxide

  - add aluminium powder

  - test using damp litmus paper

  - if litmus paper turns blue, is present
• Halide testing:

  • add silver nitrate (AgNO3) to solutions with halide ions to form a coloured precipitate
  • displacement reaction
  • Chlorine > turns white
  • Bromine > turns cream
  • Iodine > turns yellow
• Certain metals have coloured flames because:

  • sample heated> electrons excited so they jump up energy level
  • As they drop back down, they emit energy corresponding to a certain wavelength of light
  • Electrons are always in certain energy levels so their movement is always the same meaning the wavelength of light (and colour) emitted is always the same
  • An emission spectra can be used
• Metal hydroxides

  Aluminium, calcium and magnesium ions form a white precipitate with NaOH.

  Only aluminium's precipitate dissolves when excess NaOH is added.

  • Iron(II) produces a green precipitate
  • Iron(III) produces a brown precipitate
  • equations: e.g. Cu* + 20H -> Cu(OH)2
  • you need as many OH ions as the charge on the metal ion
  • the Na from the NaOH and whatever the metal ion was bonded to will react to form a compound together: e.g. CuCl, + 2NaOH →> Cu(OH), + 2NaCl