Chemistry, Third edition

19 · SEPARATING MIXTURES

Ammonium chloride is another example of a solid that is saidto sublime, but this

is untrue; it actually decomposes when heated and reforms on a cold surface:

NH 4 Cl(s)\==\NH 3 (g) + HCl(g)

Steam distillation

Steam distillation is used to separate volatile organic substances that are immiscible

with water from a mixture of involatile substances. It makes use of the fact that the

total vapour pressure of two immiscibleliquids is the sum of the vapour pressures of

the pure liquid components, so that the boiling point of the mixture is lower than

that of either of the components. This is useful for separating organic compounds

that decompose at temperatures near their boiling point and therefore cannot be

separated from a mixture by ordinary distillation. Steam from a steam generator

is passed into the impure mixture. Vapours of the desired organic component and

water condense in the condenser and are collected in the receiving flask (Fig. 19.12).

The organic component can be separated from the water layer using a separating

funnel.

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Fig. 19.12Steam distillation.

Ion exchange

Ion exchangersremove ions from a solution and replace them with other ions from

a solid phase. An ion exchanger is a column filled with small insoluble beads made of

ion exchange resin (Fig. 19.13). The beads have ions loosely bound to them which

are exchangeable with ions in a solution when it is passed through the column. The

ions in the solution are replaced with the ions which were originally bound to the

resin. After a time, the resin becomes exhausted as all its exchangeable ions have

been replaced by ions from the solution. At this point the resin has to be regenerated

and this is usually done by passing a concentrated solution of the ions which were

originally bound to the resin through the column. Ions in the concentrated solution

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