A few drops of dilute sodium hydroxide solution react to form a white precipitate with aluminium ions, calcium ions and magnesium ions. However, if excess sodium hydroxide solution is added ...

Carbonate ions also produce a white precipitate with barium chloride solution. The acid reacts with any carbonate ions present. This removes them, so stopping them giving a false positive result.

Also, the calcium hydroxide produced forms a white solid in the solution which is different from the sodium or potassium. The demonstration shown in this video is very dangerous. Do not attempt to ...