Lesson 11.6

We used sodium hydroxide and ammonia to identify positive ions.

We can carry out test to identify negative ions. Negative ions are called anions. When we have completed all our tests for both

cations and anions, we can identify unknown compound.

The whole process of finding out what elements are present in a compound is called qualitative analysis.

We call the simple ions of the Group VII elements halides.

Halides can be identified using aqueous silver nitrate.

The procedure is as follow.To a small volume of the halide solution in a test tube:1. add an equal volume of dilute nitric acid.2. add a few drops of aqueous silver nitrate.3. observe the colour of the precipitate.

Chlorides give a white precipitate. Bromides give a cream precipitate. Iodides give a pale yellow precipitate.

The precipitates are silver halides, for example:Ag+

(aq) + Br-(aq) → AgBr(s)

silver ions + bromide ions → silver bromide

In the presence of sunlight the silver chloride goes greyish-purple very quickly, and silver bromide goes greyish-purple slowly.

This are photochemical reactions (light-sensitive reactions)

Add dilute acid to the unknown compound (either a solid or a solution).

If a carbonate is present, we will see effervescence (bubbles of gas).

We test to see if the gas given off is carbon dioxide using limewater.

The identification of nitrates makes use of the test for ammonia.

The procedure is:1. Put an aqueous solution of the unknown

compound into a test tube.2. Add aqueous sodium hydroxide, then aluminium

foil and warm gently.3. Test the gas given off with a piece of damp red

litmus paper placed at the mouth of the test tube.

4. If ammonia is given off, the litmus paper will turn blue. So the compound is likely to be nitrate.

Barium chloride or barium nitrate solution is used to test for sulphates.

The procedure is:1. Put an aqueous solution of the unknown

compound into a test tube.2. Add an equal volume of dilute hydrochloric acid.3. Add an aqueous solution of a soluble barium

salt. This can be barium chloride or barium nitrate.

4. If a white precipitate is formed, the compound is sulphate.

Ba2+(aq) + SO4

2-(aq) → BaSO4(s)

barium ions + sulphate ions → barium sulphate (white precipitate)