Chemistry, Third edition

352 19 · SEPARATING MIXTURES

Solid dissolved in a liquid (in solution)

The solid cannot be separated from the solvent by filtering or centrifuging, because

now the solid particles are too small.

The solvent can be removed from the solid by evaporation. The solution is heated

until the solvent boils off, leaving the solid behind (Fig. 19.5). This is often used for sep-

arating salt from salt solution. However, there are disadvantages with this method. If

impurities are in the solvent they will be left behind and contaminate the solid. Also,

some solids may decompose during heating to dryness.

A better method of recovering a solid from a solution is crystallization. The solu-

tion is heated and some of the solvent evaporates, so the solution becomes more con-

centrated. The concentrated solution is then left to cool and the solid crystallizes out

(Fig. 19.6), with impurities tending to remain in solution. The crystals can then be

separated from the remaining solution by filtration.

Fig. 19.5Solvent removal by evaporation. Fig. 19.6Crystallization.

Separation of salt

from seawater

Seawater contains a great

deal of salt. Describe how

you would separate a

sample of salt from

seawater. Do you think the

sample would be pure

NaCl?

Exercise 19A

Fig. 19.7Simple distillation.

In order to recover the solvent from a solution, distillationis used. The solution is

heated in a flask, until it boils, then the solvent vapour is passed through a condenser

where it is cooled and condensed. The apparatus is shown in Fig. 19.7. The condenser

consists of a glass tube surrounded by a glass jacket through which cold water is

circulated. In this example, steam from the boiling salt solution passes through the

condenser and condenses into water which drips into the beaker. Salt is left behind

in the flask.