C3 -Test for positive ions

C3 -Test for positive ions

1. Some metals burn with a characteristic colour

   • Lithium - crimson
   • Sodium - yellow
   • Potassium - lilac
   • Calcium - red
   • Barium - green
   • To test a compound, dip a nichrome (clean) wire loop into a sample of the compound and put it in the clear blue part of the Bunsen flame. Make sure it is clean by dipping it into hydrochloric acid and rinsing it with distilled water.

2. Some metal ions form a coloured precipitate with NaOH

   • Many metal hydroxides are insoluable and precipitate out of solution when formed. Some have a characteristic colour
   • Add a few drops of sodium hydroxide solution to the solution of the mystery compound. If you get a coloured compound you can tell which metal it was.
   • Calcium - white (Ca2+ + 2OH- = Ca(0H)2
   • Copper(II) - blue (Cu2+ + 2OH- = Cu(0H)2
   • Iron(II) - green (Fe2+ + 2OH- = Fe0H)2
   • Iron(III) - brown (Fe3+ + 3OH- = Fe(0H)3
   • Aluminium - White at first then redissolves in excess NaOH to form a colourless solution. (Al3+ + 3OH- = Al(0H)3 then Al(OH)3+ OH- = Al(OH)4
   • Magnesium - White (Mg2+ + 2OH- = Mg(0H)2)