Chemistry, Third edition

6 · REACTIONS OF IONS IN SOLUTION

Producing ions in water by chemical

reaction

The dissolution of an ionic salt is an example of physical change, since the ions are

present in both solid salt and in solution. By definition, covalent substances contain

no ions, but some covalent substances may produceions as a result of their chemical

reaction with water. The presence of such ions may be confirmed by testing the solu-

tion to see if it conducts electricity, but it is the new reactions of the solution – the

reactions of the ions produced – that conclusively prove that chemical change has

taken place. We now look at three important examples: the reactions with water of

hydrogen chloride, pure sulfuric acid and ammonia.

Reaction of hydrogen chloride with water

Hydrogen chloride (HCl) is a gas at room temperature. The HCl molecule is covalent

but it reacts with water producing a mixture of hydroniumand chloride ions that is

commonly referred to as hydrochloric acid (Fig. 6.2):

HCl(g)H 2 O(l)H 3 O(aq)Cl(aq)

hydronium ion

The hydronium ion is a hydrated hydrogen ion, and is often abbreviated as

H(aq). The reaction of HCl(g) with water may alternatively be written as

H 2 O

HCl(g)H(aq)Cl(aq)

6.3

86

Solubilities

(i)iiWhich zinc salts are insoluble in water?

(ii)iiName two compounds which when dissolved in water will produce a precipitate of barium

sulfate.

(iii)Name two compounds which when dissolved in water will produce a precipitate of iron(II)

hydroxide.

Exercise 6C

Hydrogen

chloride

Water turns into

hydrochloric acid

(a)

(b)

HCl (g)

Cl Cl

+

H +

++

O +

H

H

O

H H

H

Cl HOH 2 Cl– H 3 O+

Fig. 6.2Making hydrochloric acid.

(a) Hydrogen chloride gas from a cylinder

is brought into contact with water. A

funnel maximizes the contact between

gas and liquid. At room temperature,

100 g of water will hold up to 37 g of

HCl(g). (b) Shows the reaction of water

and HCl(g) on a molecular scale.