Chemistry, Third edition

SEPARATING TWO LIQUIDS 353

BOX 19.2

Recrystallization

Recrystallizationis used to purify a product.

Many chemical preparations give ‘dirty’

products in which the product is

contaminated with, for example, excess

reactants or products from side reactions. A

solvent is found that dissolves the desired

product when the solvent is hot, but the

product is not soluble when the solvent is

cold. This choice of solvent is the difficult part

and involves some ‘trial and error’

experiments. The impure product is dissolved

in the minimumof the hot solvent and the hot

solution is filtered very quickly to remove

insoluble impurities before the mixture cools

down – Buchner filtration (using warm

apparatus) is useful here. The filtered

solution is allowed to cool and the product

crystallizes out. Any soluble impurities (since

they are present at low concentration) tend to

remain in solution and the desired product

crystallizes out in a more pure form. If the

product is required to be very pure, several

recrystallizations may be necessary.

Marie Curie, Nobel prize discoverer of the

elements radium and polonium, carried out

laborious recrystallization experiments in order

to eventually isolate very small amounts of a

radium compound from the uranium ore pitch-

blende, obtained from Bohemian mines. The

work was carried out in an old shed and took

years of effort – the ore had to be crushed and

stirred with various solvents to remove the

many impurities present. Eventually, in 1902,

about 0.1 g of radium chloride (RaCl 2 ) was

obtained from several tonnes of ore. The

radioactive compound ionized the surrounding

air and glowed in the dark!

Marie Sklodowska Curie, discoverer of radium.

She eventually died of radiation poisoning.

Separating two liquids

Two liquids may be immiscible(they do not mix and two layers are formed) or

miscible(they mix completely).

Immiscible liquids

The liquids can be separated using a separating funnel. For example, consider

the separation of oil and water. This mixture will separate out so that the oil floats on

top of the water. If the mixture is poured into a separating funnel (Fig. 19.8) and

allowed to settle, two layers are formed. The tap is opened to let the water run out

and closed before the oil layer runs throught the tap. In this way, the two liquids are

separated.

19.2

Fig. 19.8A separating funnel.