Chemistry, Third edition

19.1.What technique would you use to separate the following?

(i) Solid potassium chloride from an aqueous solution of

potassium chloride.

(ii) Pure water from lemonade.

(iii) Hexane from crude oil.

(iv) Oxygen from liquid air.

(v) The colouring substance in a coloured drink.

(vi) Petrol from petrol and water.

(vii)Salt from a mixture of salt and chalk.

19.2.A green substance in grass dissolves in propanone (ace-

tone). Describe how you would show that this green pigment

contains more than one substance.

19.3.How would you separate a fine mixture of ammonium

chloride and potassium chloride solids?

19.4.Screened methyl orange is a mixture of dyes which are

soluble in ethanol:

(i)What techniques could you use to show that the substance

is a mixture?

Revision questions

364 19 · SEPARATING MIXTURES

peaks – A – is a reference material, injected along with the sample (in this case air,

from an air bubble in the syringe). Two components, B and C have separated out

from the mixture. The retention time for B, for example, is the time taken for the

recorder to mark from the tip of the reference compound (A) to the tip of peak B (tB).

A summary of separation techniques is shown in Table 19.1.

BOX 19.6

Gas chromatography–mass spectrometry (GC–MS)

It is possible to ‘add on’ another

technique to analyse the components

once they are separated out from a

mixture using GC. GC–MS combines gas

chromatography with mass spectrometry

(MS). The previously unknown organic

components of a mixture are separated

by GC, then passed into a MS unit

where they are ionized and produce a

characteristic fragmentation pattern. The

fragmentation pattern of a substance

can be used as a ‘fingerprint’ for that

substance. The fingerprint of each

component can be compared with a

computer library of fingerprints of

different substances, hopefully leading

to the identification of the components

of the original mixture. For example, this

technique has been successfully

employed in identifying trace amounts of

drugs in the blood of athletes.

Table 19.1Separation techniques

Mixture Separation technique

Insoluble solid in solvent filtration (or, if a fine precipitate, centrifugation)

Solution of solid in liquid evaporation or (better) crystallization to recover

solid; distillation to recover liquid

Miscible mixture of liquids fractional distillation

Immiscible mixture of liquids separating funnel

Mixture of two solids, one of which sublimes sublimation

An organic substance that is immiscible with steam distillation

water from a mixture of involatile substances

Mixture of substances in solution paper chromatography or TLC for identification;

column chromatography to collect the substances

Removal of ions (e.g. calcium or magnesium) ion exchange

from aqueous solution

Separation and identification of a mixture of gas chromatography

volatile liquids

Separation of organic components from an solvent extraction

aqueous mixture