Separation by Distillation (College Board AP® Chemistry)

Separation by Distillation

Principles of Distillation

• A liquid solution is mixture of two miscible liquids that cannot be separated through filtration, therefore, an alternative separation technique must be used

• Distillation is a technique that exploits the differences in boiling points between the components of a liquid solution

• The boiling point of the liquids is influenced by the intermolecular forces within the molecules of the substance

  • 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 in simple 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. When the vapor pressure of one of the components of the mixture is equal to the atmospheric pressure, it will boil. This is the most volatile compound and it vaporizes first because of its weak intermolecular forces

4. The vapor passes through the condenser, where it cools down, turning itself into a liquid again

5. As the vapor condenses, the component with the stronger intermolecular forces, remains in the round bottomed flask

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

• For water and ethanol, ethanol has a boiling point of 78 ºC and water of 100 ºC. The mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distills out of the mixture and condenses into the beaker

• The temperature should not reach 100 ºC, to prevent water molecules to evaporate

• The fractional column is not necessary if there is a big difference between the boiling points of the components from the mixture

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