Chemistry, Third edition

PRODUCING IONS IN WATER BY CHEMICAL REACTION 89

bubbling the gas through the acid contained in a glass bottle. However, the greed of

the acid for water also means that it is extremely corrosive to the skin and eyes.

Use of concentrated sulfuric acid in the brown ring test for nitrates

Concentrated sulfuric acid is used to identify the nitrate ion (NO 3 ), in a test called

thebrown ring test. (Another test for nitrates is described on page 116.) This

involves the careful addition of concentrated sulfuric acid to a solution of the sus-

pected nitrate. A solution of iron(II) sulfate is then slowly added to the mixture. If a

nitrate is present, the mixture produces nitric oxide gas (NO) which reacts with

Fe^2 (aq) to produce a brown ring of Fe(NO)^2 at the junction of the acid mixture

and the FeSO 4 solution:

Fe^2 (aq)NO(aq)Fe(NO)^2 (aq)

Reactions of dilute sulfuric acid

The reactions of dilute sulfuric acid are really the reactions of the H(aq) and

SO 42 (aq) ions. The H(aq) ions provide the acidic properties. The presence of a

sulfate ion is confirmed by adding barium (Ba^2 (aq)) ions, when a dense white pre-

cipitate of barium sulfate is observed:

Ba^2 (aq)SO 42 (aq)BaSO 4 (s)

The usual source of barium ions is barium chloride solution. In order to make

this test specific for SO 42 (aq) ions, hydrochloric acid is added to prevent barium

sulfite or barium carbonate from precipitating. (See Exercise 6G.)

Reactions of barium chloride

Which of the following would give a white precipitate when added to a mixture of barium

chloride and hydrochloric acid?

(i) potassium sulfate

(ii)magnesium carbonate

(iii)sodium chloride.

Exercise 6G

Reaction of ammonia gas with water

Ammonia gas (NH 3 (g)) is extremely soluble in water. The reaction is

NH 3 (g)H 2 O(l) NH 4 (aq)OH(aq)

ammonium ion

Ammonia solution is sometimes (incorrectly) called ammonium hydroxide. A

saturated solution at room temperature contains about 28% of ammonia by mass.

The density of such a solution is 0.88 g cm^3. For this reason a saturated ammonia

solution is referred to as ‘88’ ammonia.

The OH(aq) ion is the hydroxide ion. It is this ion that gives solutions of bases

their characteristic reactions; for example, it causes red litmus to turn blue.

The symbol reflects the fact that it is impossible to convert all the ammonia

molecules to ammonium and hydroxide ions. In other words, the reaction is a

reversibleorequilibriumreaction.