Topic: Separations And Purifications

Distillation is a purification method where the components of a liquid mixture are vaporized and then condensed and isolated at different boiling points.

Distillation is an effective purification method to separate mixtures comprised of two or more pure liquids. During distillation, the components of a liquid mixture are vaporized and then condensed and isolated. In simple distillation, a mixture is heated and the most volatile component vaporizes (evaporates) first, at the lowest temperature. The vapor passes through a cooled tube (a condenser), where it condenses back into its liquid state. The pure, condensed liquid that is collected is called the distillate.

Two types of distillation can be performed depending on the complexity of the mixture:

Simple distillation involves the separation of two chemicals in a mixture on a small scale. For example, if we had an organic mixture of hexane and toluene and we heated them, both would evaporate but hexane would condense first as it has a lower boiling point. This could then be collected in a conical flask as a distillate.

Fractional distillation is used when there is a more complex mixture often with many fractions with not just fixed boiling points but ranges of temperatures where they boil. The equipment is more complex as well: a fractioning column filled with beads is used to condense different compounds at different temperatures more efficiently. This allows for obtaining greater purity in the end. Vacuum distillation uses a vacuum to lower the boiling point of the components so lower temperatures can be used for more volatile substances.

 Different strategies can be employed to ensure the safety and efficacy of the distillation procedure. One problem that can occur is superheating, a phenomenon in which increased surface tension allows a liquid to exceed its boiling point without actually boiling. If the tension is broken, then violent flash boiling can occur. To prevent this, boiling chips are often added as a nucleation surface to encourage the formation of air bubbles during boiling.

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Key Points

• Distillation is a purification process where the components of a liquid mixture are vaporized and then condensed and isolated at different boiling points.

Key Terms

Condensation: when a gas turns into a liquid

Evaporation: when a liquid turns into a gas

Fraction: a component of a mixture

Distillate: separated product of distillation

Fractional distillation: distillation of a complex mixture

Superheating: when liquid exceeds its boiling point without boiling

Boiling chips: added to distillation mixtures as a nucleation surface for air bubbles when boiling

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