Chemistry, Third edition

96 6 · REACTIONS OF IONS IN SOLUTION

2. Neutralization of acids

The OH(aq) ion neutralizes acids giving a salt and water only.

3. Reaction with NH 4 +ion

Ammonia gas is produced:

heat

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

4. Reaction with some metal ions in solution: the use of precipitation

in analysis

The hydroxide ion causes insoluble metal hydroxides to precipitate out of solution.

For example, if we add a few drops of sodium hydroxide solution to (blue)

copper(II) sulfate solution, a beautiful blue precipitate of copper(II) hydroxide is

observed:

Cu^2 (aq)2OH(aq)Cu(OH) 2 (s)

blue

precipitate

The Cu^2 (aq) comes from the copper(II) sulfate and the OH(aq) comes from the

sodium hydroxide. Similar reactions take place with other metal ions:

Zn^2 (aq)2OH(aq)Zn(OH) 2 (s)

zinc ion white precipitate

(colourless)

Al^3 (aq)3OH(aq)Al(OH) 3 (s)

aluminium ion white precipitate

(colourless)

Fe^2 (aq)2OH(aq)Fe(OH) 2 (s)

iron(II) ion white precipitate which almost

(pale-green colour) instantaneously turns green

Fe^3 (aq)3OH(aq)Fe(OH) 3 (s)

iron(III) ion reddish-brown precipitate

(yellow/brown colour)

Pb^2 (aq)2OH(aq)Pb(OH) 2 (s)

lead(II) ion white precipitate

(colourless)

Mg^2 (aq)2OH(aq)Mg(OH) 2 (s)

magnesium ion white precipitate

(colourless)

Zinc, aluminium and lead(II) hydroxides will dissolve to make clear solutions if

we add excess sodium hydroxide solution. This happens because all three hydroxides

areamphoteric(see Box 6.2). This effect is useful because it helps to distinguish be-

tween magnesium hydroxide (whose precipitate is insoluble in excess hydroxide)

and zinc, aluminium and lead(II) hydroxides (whose precipitates are soluble in

excess hydroxide).

Since zinc, aluminium and lead(II) hydroxides are white, additional tests are

needed to distinguish between them. A quick identification test for lead(II) ions

depends upon the fact that lead(II) sulfate is insoluble in water, whereas aluminium

and zinc sulfates are soluble. Addition of dilute sulfuric acid to lead(II) ions will

therefore precipitate the lead(II) sulfate:

Pb^2 (aq)SO 42 (aq)PbSO 4 (s)

white precipitate