1. Identifying Cations

2. Introduction Cations are The ions of metals
They have a positive charge
e.g. Na+ Ca2+ Cr3+
They are called cations because they are attracted to the opposite charge of the cathode.
NH4+ [ammonium] is also a cation.

3. Apparatus
Test tubes
Solutions of salts containing various cations [Cu2+, Fe2+, Fe3+, Zn2+, Ca2+, Al3+.]
sodium hydroxide solution [NaOH]
ammonium hydroxide solution [NH4OH]

4. Method Look at the solution
Colour will often give a clue to the ions present.
Take a small sample
Add a small amount of NaOH solution.
Add NaOH till it is in excess
Note what happens in each case.
Take a fresh sample of cation and repeat using ammonium hydroxide solution [NH4OH]

5. Cu2+ - Copper(II) The solution is blue
this suggests that it contains copper ions
When NaOH was added a pale blue precipitate of copper hydroxide formed
Cu2+(aq) OH-(aq) = Cu(OH)2(s)
The precipitate remains when excess sodium hydroxide is added

6. Cu2+ - Copper(II) Take a fresh sample
When ammonia solution is added a pale blue precipitate of copper hydroxide is also formed.
When excess ammonia solution is added the blue precipitate re-dissolves and forms a royal blue solution
This if final confirmation that it is copper(II)

7. Fe2+ - Iron(II) The solution is pale green
This suggest that it contains iron(II)
When NaOH was added a dirty green precipitate of iron(II) hydroxide formed
Fe2+(aq) OH-(aq) = Fe(OH)2(s)
The precipitate remains when excess sodium hydroxide is added.
This confirms it is iron(II)

8. Fe2+ - Iron(II) Take a fresh sample
When ammonia solution is added a dirty green precipitate of Iron(II)hydroxide is also formed.
It does not re-dissolve in excess ammonia solution.
This confirms it is iron(II)

9. Fe3+ - Iron(III) The solution is brown/ yellow
This suggest that it contains iron(III)
When NaOH was added a dirty brown precipitate of iron(III) hydroxide formed
Fe3+(aq) OH-(aq) = Fe(OH)3(s)
The precipitate remains when excess sodium hydroxide is added.
This confirms it is iron(III)

10. Fe3+ - Iron(III) Take a fresh sample
When ammonia solution is added a dirty brown precipitate of Iron(III)hydroxide is also formed.
It does not re-dissolve in excess ammonia solution.
This confirms it is iron(III)

11. Aluminium Al3+ Colourless no inference Add sodium hydroxide
White precipitate forms
Al3+ (aq) + 3 OH- (aq) = Al(OH)3(s)
precipitate does re-dissolve in excess NaOH

12. Aluminium Al3+ Take a fresh sample Add ammonium hydroxide
White precipitate forms
precipitate does not re-dissolve in excess NH4OH

13. Zinc Zn2+ Colourless no inference Add sodium hydroxide
White precipitate forms
Zn2+ (aq) + 2 OH- (aq) = Zn(OH)2(s)
precipitate does re-dissolve in excess NaOH

14. Zinc Zn2+ Take a fresh sample Add ammonium hydroxide
White precipitate forms
precipitate does re-dissolve in excess NH4OH

15. Calcium Ca2+ Colourless no inference Add sodium hydroxide
White precipitate forms
Ca2+(aq) + 2 OH- (aq) = Ca(OH)2(s)
Does not re-dissolve in excess

16. Calcium Ca2+ Take a fresh sample Add ammonium hydroxide
No precipitate forms