For example, ethanol boils at 78 °C whereas water boils at 100 °C. When a mixture of the two is heated, ethanol and water vapours enter the fractionating column. Glass beads in the column provide a large surface area for condensation (Figure 2.16b). Evaporation and condensation take place many times as the vapours rise up the column. Ethanol passes through the condenser first as the temperature of the column is raised above its boiling point. Water condenses in the column and flows back into the flask because the temperature of the column is below its boiling point of 100 °C. The temperature on the thermometer stays at 78 °C until all the ethanol has distilled over. Only then does the temperature on the thermometer rise to 100 °C and the water distil over. By watching the temperature carefully, the two liquids (fractions) can be collected separately. Fractional distillation is used to separate any solution containing liquids with different boiling points. The liquid in the mixture with the lowest boiling point (the most volatile) distils over first. The final liquid to distil over is the one with the highest boiling point (the least volatile). Fractional distillation can be adapted as a continuous process and is used industrially to separate: ◆ the various fractions from petroleum (page 276), ◆ the different gases from liquid air (page 7).
Study tip In fractional distillation remember that it is the liquid with the lowest boiling point that distils over first.
Separating two or more dissolved solids in solution can be carried out by chromatography. There are several types of chromatography, but they all follow the same basic principles. Paper chromatography is probably the simplest form to set up and is very useful if we want to analyse the substances present in a solution. For example, it can tell us whether a solution has become contaminated. This can be very important because contamination of food or drinking water, for instance, may be dangerous to our health. A drop of concentrated solution is usually placed on a pencil line near the bottom edge of a strip of chromatography paper. The paper is then dipped in the solvent. The level of the solvent must start below the sample. Figure 2.17 (overleaf) shows the process in action. Many different solvents are used in chromatography. Water and organic solvents (carbon-containing solvents) such as ethanol, ethanoic acid solution and propanone are common. Organic solvents are useful because they dissolve many substances that are insoluble in water. When an organic solvent is used, the process is carried out in a tank with a lid to stop the solvent evaporating.
Activity 2.4 Investigation of food dyes by chromatography This experiment involves testing some food colours with paper chromatography to find out if they are pure colours or mixtures of several dyes. These food colours are used in cake making, for instance, and there is quite a wide range of permitted colours readily available. A worksheet is included on the CD-ROM. Adaptations of this experiment are given in the Notes on Activities for teachers/technicians.
Activity 2.3 Distillation of mixtures In this experiment, several mixtures will be separated using different types of distillation apparatus, including a microscale distillation apparatus. A worksheet is included on the CD-ROM.
The substances separate according to their solubility in the solvent. As the solvent moves up the paper, the substances are carried with it and begin to separate. The substance that is most soluble moves fastest up the paper. An insoluble substance would remain at the origin. The run is stopped just before the solvent front reaches the top of the paper.
Original material © Cambridge University Press 2014
Chapter 2: The nature of matter
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