Thursday, May 20, 2010


6.1: Simple Distillation

Fractional distillation separates a pure liquid from a solution.

In distillation, the liquid is changed from a vapour by boiling. The vapour is pure as other substances are left behind. The vapour is then cooled. It condenses to a pure liquid which is called the distillate. Distillation can be used to obtain a pure solvent from a solution of a solute. An exmaple of a simple distillation is the distillation of seawater to obtain pure water. The seawater in the flask is heated to boiling. Pure water vapour is produced. In the condenser, the steam is cooled to form pure liquid water. The salt remains in the flask.

6.2: Desalination?

In countries with few rivers or lakers to provide an adequate source of water, pure water is obtained from seawater. This is called desalination. The distillation of seawater is one way that pure water is obtained. Distillation plants exist in about 120 countries around the world. The disadvantage with the use of distillation is cost. The main fuel used to heat the water in the distillation process is oil, and as alot of oil is required, the pure water produced is very expensive.

6.3: Fractional Distillation

Fractional distillation separates a mixture of miscible liquids with widely differing boiling points.
Miscible liquids are completely soluble in each other. This means two liquids are mixed to form one liquid. A mixture of miscible liquids can be separated by fractional distillation. This is done with the help of a fractionating column. For example, water and alcohol are miscible and have boiling points of 100 and 78 respectively. The fractioning column used is normaly packed with glass beads or some other unreactive substances. This provides a large surface area for condesation. When the flask is heated, the vapour coming off the mixture wil lcontain both ethanol and water molecules. However, it will be richer in ethanol molecules as these have the lower boiling point of the two.

At first, this vapour just condenses on the cold fractionating column, but as this column warms up, molecules in the vapour state rise further before condensing. As we go up the column, the temperature becomes lower and so the proportion of the ethanol molecules with the lower boiling point increases. When the temperature at the top of the column reaches 78 degrees celsuis, molecules of ethanol can survive as vapour and these pass over in the Liebig condenser. Water molecules with the higher boiling point condense in the fractionating column and fall back into the flask. This continues until most of the ethanol is boiled off. When the temperature at the top of the column rises to 100 degrees celsius, water passes into the condenser. it can be collected in a different receiver.