4.2.3 tests for aqueous cations & anions

• Metal ions can be found in aqueous solutions of metal compounds. These ions can precipitate out of solution when reacted with sodium hydroxide solution (NaOH) to produce insoluble solids called precipitates.
• White precipitates:

  • Magnesium hydroxide
  • Calcium hydroxide
  • Aluminium hydroxide
• Magnesium ions (Mg2+) form the white precipitate magnesium hydroxide

  • Mg(2+)(aq) + 2OH(-)(aq) -> Mg(OH)2(s)
• Aluminium ions (Al3+) form the white precipitate aluminium hydroxide.

  • Al(3+)(aq) + 3OH(-)(aq) -> Al(OH)3(s)

  Unlike the other two white precipitates, aluminium hydroxide can be dissolved in excess NaOH. This produces a colourless solution
• Calcium ions (Ca2+) form the white precipitate calcium hydroxide

  • Ca(2+)(aq) + 2OH(-)(aq) -> Ca(OH)2(s)
• Iron(II) ions (Fe2+) form the green precipitate Iron(II) hydroxide
• Iron(III) ions (Fe3+) form the brown precipitate Iron(III) hydroxide
• Copper(II) ions (Cu2+) form the blue precipitate Copper(II) hydroxide
• Testing for carbonates - hydrochloric acid + carbonate:

  • A reaction between a carbonate and a dilute acid gives a salt, carbon dioxide and water.
  • E.g. Calcium carbonate + hydrochloric acid → calcium chloride + carbon dioxide + water
  • As carbon dioxide is produced, we can use the standard carbon dioxide test to work out if an unknown substance is a carbonate.
  • Bubble the gas through limewater. if the solution turns cloudy, the gas is carbon dioxide.
• Carbonates contain CO3(2-) ions
• The products of a reaction between a carbonate and a dilute acid:

  • carbon dioxide
  • water
  • a salt