Chemistry, Third edition

SEPARATING A SOLID FROM A LIQUID 351

BOX 19.1

Folding a filter paper

The diagrams in Fig. 19.2 show how a filter paper is folded for gravity filtration:

Gravity filtration has the disadvantage of being rather slow. The process of filtration is speeded

up using vacuum filtration, but more sophisticated equipment is required. A vacuum is used to

‘pull’ the liquid through the filter paper (Fig. 19.3).

A special funnel (called a Buchner funnel) is used which is connected, via a rubber seal, to a

thick-walled flask with a side arm. A pump is attached to the side arm of the flask, and when

the vacuum is applied the solution in the funnel is filtered rapidly. The filter paper lies flat inside

the bottom of the funnel. The liquid is sucked through the filter paper, leaving insoluble solid

behind. If the liquid is volatile, for example ethoxyethane (diethyl ether), sucking air through the

filter paper for a few minutes after all the liquid has been pulled through is sufficient to dry the

solid.

Fig. 19.2Folding a filter paper.

Fig. 19.3Vacuum filtration.

Centrifuge

Solid

Clear

liquid

Pipette

Fine

suspension

Fig. 19.4Removing the liquid from a centrifuge tube.