Chemistry

Matter in Our Surroundings

To Separate a Mixture of Alcohol and Water

Alcohol (or ethanol), and water are miscible liquids. The boiling point of alcohol is 78°C and the boiling point of water is 100°C. Since the boiling points of alcohol and water are different, therefore, a mixture of alcohol and water can be separated by fractional distillation. The apparatus used for fractional distillation of alcohol and water mixture is shown in Figure 81.
The mixture of alcohol and water is heated in a distillation flask fitted with a fractionating column (see Figure 81). When the mixture is heated, both alcohol and water form vapours as their boiling points approach.The alcohol vapour and water vapour rise up in the fractionating column. The upper part of the fractionating column is cooler, so as the hot vapours rise up in the column,they get cooled, condense and trickle back into the distillation flask. As the experiment goes on, the fractionating column warms up by the heat released by the condensed vapours. After some time, a temperature gradient is created in the fractionating column, the temperature at the top of the column being much less than at its bottom.
When the temperature at the top of the fractionating column reaches 78°C (which is the boiling point of alcohol),then alcohol vapour passes into the condenser, gets cooled and collects in a beaker kept at the other end of the condenser(see Figure 81). The alcohol-water mixture is kept boiling at such a rate that the thermometer shows the boiling point of alcohol (78°C). In this way, all the alcohol distils over and gets separated. It is collected as the first fraction.
Please note that when thermometer is showing the boiling point of alcohol (78°C), no water distils over. This is due to the fact that the temperature at the top of the fractionating column (78°C) is much less than the boiling point of water, which is 100°C. So, all the water vapour condenses before reaching the top of the fractionating column and falls back into the distillation flask.
Having collected the alcohol fraction, the flask is heated more strongly so that the thermometer shows a temperature of 100°C which is the boiling point of water. When the temperature at the top of the fractionating column becomes 100°C, water vapour passes into the condenser, gets cooled and condenses. This pure water is collected in another beaker as the second fraction. Heating is continued till all the water distils over.In this way, the alcohol-water mixture has been separated into two fractions boiling at 78°C and 100°C, respectively.
It is clear from the above discussion that by using fractional distillation, a mixture of two (or more) miscible liquids can be separated completely. Fractional distillation separates the various liquids according to their boiling points : the more volatile liquid (having lower boiling point) distils over first, and the less volatile liquid (having higher boiling point) distils over later. Acetone and water are miscible liquids.The boiling point of acetone is 56°C and the boiling point of water is 100°C. So, a mixture of acetone and water can be separated by fractional distillation. Please note that a mixture of more than two miscible liquids can also be separated by fractional distillation (provided they all have different boiling points).For example, a mixture of acetone, alcohol and water can be separated by fractional distillation.

Notes

Matter in Our Surroundings - Notes

1. 3. Non-Metals are Bad Conductors of Heat and Electricity.