Colourless Cations

Most of the cations we need to identify are colourless: Ag⁺, Mg²⁺, Pb²⁺, Ba²⁺, Zn²⁺, Al³⁺, Na⁺ and NH₄⁺.

This makes them a bit tougher to identify in an unknown solution.

Step One

The easiest way to start is to add a few drops of NaOH:
Muddy Brown Precipitate = Ag⁺
No Precipiatate = Na⁺ or NH₄⁺  (you will not be asked to tell these two ions apart in an assessment)
White Precipitate = any of the others

Step Two

If a white precipitate has formed, add another 2-3 mL of NaOH:
Precipitate Remains = Ba²⁺ or Mg²⁺
Precipitate Disappears (redissolves) =  Pb²⁺, Zn²⁺or Al³⁺

Barium or Magnesium?

Collect a new sample and add about 1 mL of sulfuric acid. The sulfate ions will form a white precipitate with barium ions, but not with magnesium:

White Precipitate =  Ba²⁺

No Precipitate =  Mg²⁺

Lead, Zinc or Aluminium?

Collect a new sample and add a few drops of potassium iodide (KI):

Yellow Precipitate =  Pb²⁺

No Precipitate =  Al³⁺or Zn²⁺

Collect a new sample if it is not  Pb²⁺
Add excess/concentrated ammonia. Both will form a white precipitate with the hydroxide ions in ammonia (it is a base), but zinc will redissolve, forming a complex ion with the ammonia molecules. Aluminium will not make this complex.
No Precipitate =  Zn²⁺
White Precipitate =  Al³⁺