Separation by Distillation (College Board AP® Chemistry): Revision Note

Separation by Distillation

Principles of distillation

• A liquid solution, such as a mixture of two miscible liquids, cannot be separated by filtration, so another separation technique must be used

• Distillation separates the components of a liquid solution using differences in the strength of their intermolecular forces, which give them different vapor pressures and boiling points

• The boiling point of each liquid depends on the strength of the intermolecular forces between its molecules

  • The stronger the intermolecular forces, the higher the boiling point of the liquid

Conducting a separation by distillation

• A distillation of a liquid solution of water-ethanol can be carried out in these steps:

1. Place the mixture in a round-bottom flask, and connect it to a fractionating column followed by a condenser

2. Apply heat to the flask to increase the vapor pressure of the components in the mixture. Using an electric heater is mandatory when flammable liquids are present in the mixture

3. The mixture boils when its total vapor pressure equals atmospheric pressure. The vapor is richer in the more volatile component, which has weaker intermolecular forces and so a higher vapor pressure

4. The vapor passes through the condenser, where it cools and condenses back to a liquid

5. Meanwhile, most of the component with the stronger intermolecular forces (water) stays in the round-bottom flask

6. Finally, the cooled liquid is collected in a separate container

• Ethanol boils at 78 °C and water at 100 °C. The vapor that reaches the top of the fractionating column is mostly ethanol, so the thermometer there reads about 78 °C while ethanol-rich liquid distills over and is collected

• Water still has a vapor pressure below 100 °C, so some water evaporates too. Keeping the temperature at the top of the column close to 78 °C keeps the collected liquid mostly ethanol

• A fractionating column is not needed if the boiling points are far apart: this is simple distillation

Method of separation by distillation

Separation of a mixture of ethanol and water is best achieved by fractional distillation when the two components are close together in boiling point or there are multiple components