Metal Hydroxide Precipitate Tests

What reagent is used to identify certain metal ions by precipitation?

Sodium hydroxide solution.

Why does sodium hydroxide form precipitates with some metal ions?

It reacts with the ions to form insoluble metal hydroxides.

What is a precipitate?

An insoluble solid formed during a reaction in solution.

What colour precipitate forms with aluminium ions?

White.

What colour precipitate forms with calcium ions?

White.

What colour precipitate forms with magnesium ions?

White.

What colour precipitate forms with copper(II) ions?

Blue.

What colour precipitate forms with iron(II) ions?

Green.

What colour precipitate forms with iron(III) ions?

Brown.

Which metal ions produce white precipitates with sodium hydroxide?

Al³⁺, Ca²⁺ and Mg²⁺.

Which metal ion produces a blue precipitate?

Cu²⁺.

Which metal ion produces a green precipitate?

Fe²⁺.

Which metal ion produces a brown precipitate?

Fe³⁺.

Why can some metal ions not be distinguished using precipitate colour alone?

Several ions form white hydroxide precipitates.

How can aluminium hydroxide be distinguished from calcium and magnesium hydroxides?

Aluminium hydroxide dissolves in excess sodium hydroxide.

What happens to calcium hydroxide precipitate in excess sodium hydroxide?

It does not dissolve.

What happens to magnesium hydroxide precipitate in excess sodium hydroxide?

It does not dissolve.

Why is sodium hydroxide added gradually during precipitation tests?

To observe precipitate formation and any changes in excess reagent.

What is the difference between a solution and a precipitate?

A solution contains dissolved particles, while a precipitate is an insoluble solid.