Which metal ions can be identified using sodium hydroxide solution?

The identification of metal ions using sodium hydroxide solution is based on the formation of precipitates or changes that occur when specific metal ions react with hydroxide ions. In the case of aluminium, calcium, and magnesium, each of these metal ions forms characteristic hydroxides when sodium hydroxide is added.

Aluminium ions react with sodium hydroxide to produce a white precipitate of aluminium hydroxide, which can dissolve in excess sodium hydroxide to form a colourless solution. Calcium ions also form a white precipitate of calcium hydroxide when reacted with sodium hydroxide, while magnesium ions produce a similarly white precipitate of magnesium hydroxide.

In contrast, other combinations listed do not yield similar observable reactions when mixed with sodium hydroxide. For example, sodium and potassium do not form precipitates with sodium hydroxide, as they are strong alkali metals. Silver and mercury, while they can react with hydroxide, do not fit in the context of simple identification with sodium hydroxide as directly as the correct answer does. Thus, the correct combination is aluminium, calcium, and magnesium, as these all form distinct precipitates, allowing for easy identification with sodium hydroxide.