distillation heading

Higher and foundation tier

Lets assume you have a mixture of three liquids in a flask. These liquids are water, which boils at 1000C, ethanol (alcohol) which boils at 780C and acetone (nail varnish remover) which boils at 560C. To separate a mixture of these liquids a process called distillation is used. Distillation is simply evaporation followed by condensation and by carefully monitoring temperatures separating a mixture of liquids with different boiling points is very easy. The image below shows the apparatus set-up in the lab.

distillation apparatus

Explaining distillation

distillation aparatus The mixtures of the three liquids are placed in a round bottomed-flask and heated gently with a Bunsen burner. A few anti-bumping granules, usually some pieces of broken ceramic pot or small plastic beads are added to the mixture of liquids. The anti-bumpng granules will ensure the liquids boil smoothly and prevent large gas bubbles forming which can cause splashing and bumping in the mixture.

As the mixture of liquids is heated the liquid with the lowest boiling point will evaporate first, in this case it is acetone, with a boiling point of 560C. The acetone vapour will rise up the flask and enter the Liebig condenser. This is simply a tube within a tube, the inner tube is kept cool by a constant stream of cold water from a tap. The water then leaves the Liebig condenser at the top, as shown in the simplified diagram opposite. The thermometer will read 560C, the boiling point of acetone. As the acetone vapour hits the inner Liebig tube it will condense and turn back into liquid acetone, which can be simply collected in a beaker, flask or test-tube.

When the temperature on the thermometer begins to rise above 560C then all the acetone has been collected and the next volatile liquid, in this case ethanol, boiling poin 780C will start to evaporate. Simply replace the collecting beaker with a new one and you can collect the liquid ethanol. The ethanol vapour as with the acetone vapour will enter the Liebig condenser and condense back into liquid ethanol. Once again when the thermometer reading starts to rise above 780C then a new beaker is required to collect the next liquid, in this case water. The thermometer this time will read 1000C as the water evaporates.

Fractional distillation

fractional distillation If the liquids to be separated have boiling points which are close together then a tall fractionating column maybe added to the round-bottomed flask, as shown opposite. This is simply a tall glass column which contains small glass like spikes or it may be filled with glass beads. However the point is that it is cooler at the top of the column than it is at the bottom. This means that the column contains a temperature gradient - cooler at top, warmer at the bottom . This means that if a mixture of vapours with slightly different boiling points enter the fractionating column from the round-bottomed flask they will condense at different heights inside the column. If a vapour condenses inside the fractionating column it will simply fall back down the column and into the round-bottomed flask. This means that if the temperature gradient inside the column is small enough it will be possible to separate vapours with boiling points which are very close together, but careful control of the temperature inside the fractionating column will be needed.

Key Points

Practice questions

Check your understanding - Distillation questions