Chemistry, Third edition

USE OF REACTIONS IN IDENTIFICATION OF IONS IN SOLUTION

Zinc and aluminium ions may be distinguished by heating a small amount of

their hydroxides on a metal spatula. Aluminium hydroxide decomposes to form

white aluminium oxide. Zinc hydroxide also decomposes into the oxide, but zinc

oxide (although white when cold) is yellow when hot.

Silver hydroxide does not exist. When hydroxide ions are added to silver ions, a

dark brown precipitate of silver oxide is formed:

2Ag(aq)2OH(aq)Ag 2 O(s)H 2 O(l)

Because potassium and sodium hydroxides are very soluble, they do not precipi-

tate when OH ions are added to sodium or potassium salts. Calcium hydroxide is

not very soluble in water, and appears as a cloudiness when a concentrated solution

of OHis mixed with a concentrated solution of calcium salts. Na(aq), K(aq)

and Ca^2 (aq) may be identified using flame tests(see pages 195 and 197).

Use of reactions in the identification of

ions in solution

We have looked at many ion reactions in this chapter, and we shall now look at how

simple test-tube experiments may be used to identify common ions. A list of the ions

which are relevant to the problems below is given in Table 6.3.

6.8

97

Example 6.2

A solution of a simple salt (AB, where A is the cation and B is the

anion) is pale green. Addition of a mixture of barium chloride

solution and dilute hydrochloric acid produces a white

precipitate. Addition of sodium hydroxide solution to AB

produces a green precipitate. Identify A and B.

Answer

The colour of the solution suggests that the iron(II) ion is present. This is

confirmed by the green precipitate with hydroxide ion. The precipitate with BaCl 2 +

HCl confirms the presence of a sulfate.

A = Fe2+(aq); B = SO 42 (aq); AB = FeSO 4

BOX 6.2

Amphoteric hydroxides

Hydroxides (like all bases) react with acids.

For example, aluminium hydroxide dissolves in

acid giving Al^3 (aq):

Al(OH) 3 (s)3H(aq)Al^3 (aq)3H 2 O(l)

colourless

However, some hydroxides will also react with

hydroxide ion if it is present in high enough

concentration. For example, with aluminium

hydroxide the reaction is:

Al(OH) 3 (s)OH(aq)[Al(OH) 4 ](aq)

colourless

In this reaction, aluminium hydroxide is acting

as an acid. Aluminium hydroxide is said to be

amphoteric, meaning that it acts like a base

or acid depending on whether excess H(aq)

or OH(aq) is present. Zinc and lead(II)

hydroxides are also amphoteric and both

react in a similar way to aluminium hydroxide.