Testing for Ions

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Summary

There is a variety of simple chemical tests we can use to find out what ions are present in a solution.

We can test for common anions (negative ions) as shown in the table below.

Anion Test

chlorideDissolve in dilute nitric acid, then a white precipitate forms with silver nitrate solution.

(The precipitate dissolves in dilute ammonia solution.)

bromideDissolve in dilute nitric acid, then a cream precipitate forms with silver nitrate solution.(The precipitate is insoluble in dilute ammonia solution, but will dissolve in concentrated

ammonia solution)

iodideDissolve in dilute nitric acid, then a pale yellow precipitate forms with silver nitratesolution. (The precipitate is insoluble in dilute and concentrated ammonia solution)

sulfateDissolve in dilute nitric acid, then a white precipitate forms with barium chloride

solution.

nitrateAdd sodium hydroxide solution and warm with powdered aluminium, then test the gas

given off (ammonia) with damp red litmus paper, which turns blue.

carbonateAdd dilute acid, then pass the carbon dioxide gas through limewater, which turns milky

(cloudy).

We can test for positive ions by adding sodium hydroxide solution and noting the colour of the

precipitate, as shown in the table below.

Cation Result of adding sodium hydroxide solution

copper(II) pale blue precipitate

iron(II) dirty green precipitate

iron(III) rusty brown precipitate

aluminium white precipitate which dissolves in excess sodium hydroxide

magnesium white precipitate

calcium white precipitate

Ammonium ions give off ammonia gas when we heat them with sodium hydroxide solution.

Some metal ions also give out coloured light when we heat them in a Bunsen flame. Their characteristic

colours are shown in the table below.

Cation Colour of flame test

lithium red (scarlet)

sodium bright yellow

potassium lilac

barium apple green

calcium brick red

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