While the solvent is evaporating, dip a glass rod into the solution from time to time. When small crystals form on the rod, take the solution off the water bath and leave it to cool.
solution in evaporating basin boiling water gauze
Figure 2.14 An evaporation method. This method should not be used if the solvent is flammable. Instead, use an electrical heating element and an oil or water bath.
Separating solutions The separation of this type of mixture is often slightly more complicated because there is no physical separation of the phases in the original mixture. The methods of separation usually depend on solubility properties or on differences in boiling point (or volatility). Separating a solid from solution in a liquid can be carried out by evaporation or crystallisation. Evaporation gives only a powder, but crystallisation can result in proper crystals. Both processes begin by evaporating away the liquid but, when crystals are needed, evaporation is stopped when the solution has been concentrated enough. Figure 2.14 shows how this can be judged and done safely. The concentrated solution is allowed to cool slowly. The crystals formed can then be filtered off and dried. Separating a liquid from a solution is usually carried out by distillation (Figure 2.15). The boiling point of
the liquid is usually very much lower than that of the dissolved solid. The liquid is more volatile than the dissolved solid and can easily be evaporated off in a distillation flask. It is condensed by passing it down a water-cooled condenser, and then collected as the distillate. Separating the liquids from a mixture of two (or more) miscible liquids is again based on the fact that the liquids will have different boiling points. However, the boiling points are closer together than for a solid-inliquid solution and fractional distillation must be used (Figure 2.16). In fractional distillation the most volatile liquid in the mixture distils over first and the least volatile liquid boils over last. a thermometer
water out condenser fractionating column (glass beads)
78 °C 79 °C 80 °C
water drips water in back into flask solution of ethanol ethanol and water electrical heater
b
thermometer water out condenser
seawater water in heat pure water Figure 2.15
30
The distillation of seawater.
Cambridge IGCSE Chemistry
Figure 2.16 a Separating a mixture of ethanol (alcohol) and water by fractional distillation. b A close-up of the glass beads in the column.
Original material © Cambridge University Press 2014