Chemistry- Test for positive ions

Dip a wire loop into some hydrochloric acid to clean it.

Put the loop into a powdered sample of the compound to be tested, then place the end in a blue Bunsen fame.

Once heated the compounds of some metals give a characteristic colour flame.

Sodium, Na⁺, gives a yellow/ orange flame

Calcium, Ca²⁺, gives a brick-red flame

Potassium, K⁺, gives a lilac flame

Copper, Cu²⁺, gives a blue-green flam

Many metal hydroxides are insoluble and precipitate out of solution when formed. Some of these hydroxides have a characteristic colour.

Add a few drops of sodium hydroxide solution to the solution of the compound you wish to identify.

If you get a coloured insoluble hydroxide, you can tell which metal was in the compound.

• Calcium, Ca²⁺, forms a white precipitate and the ionic reaction is:
  • Ca²⁺(aq) + 2OH^-(aq) -> Ca(OH)2 (s)
• Copper(II), Cu2+, forms a blue precipitate and the ionic reaction is:
  • Cu²⁺(aq) + 2OH^-(aq) -> Cu(OH)2 (s)
• Iron(II), Fe²⁺, forms a sludgy green precipitate and the ionic reaction is:
  • Fe²⁺(aq) + 2OH^-(aq) -> Fe(OH)2 (s)
• Iron, Fe³⁺, forms a reddish brown precipitate and the ionic reaction is:
  • Fe³⁺(aq) + 3OH^-(aq) -> Fe(OH)3 (s)
• Aluminium, Al³⁺, forms a white precipitate at first but then redissolves in excess NaOH to form a colourless solution and the ionic reaction is:
  • Al³⁺(aq) + 3OH^-(aq) -> Al(OH)3 (s)
  • Then: Al(OH)3 (s) + OH^-->Al(OH)4 ^-(aq)