A Level Chemistry OCR Practice Exam

What does fractional distillation primarily rely on for separating liquid mixtures?

1. Density differences

2. Boiling point differences

3. Viscosity differences

4. Solubility differences

The correct answer is: Boiling point differences

Fractional distillation primarily relies on boiling point differences to separate liquid mixtures. This technique is particularly effective when dealing with mixtures of liquids that have different boiling points. During the fractional distillation process, a mixture is heated, and as it approaches the boiling point of the components, the substance with the lower boiling point vaporizes first. The vapor then travels up a fractional column, where it can cool and condense back into liquid form, allowing for careful separation of the components based on their boiling points. The efficiency of this method hinges on the ability to discern these boiling point differences, which allows for sequential collection of the various components. This principle is especially useful in industries involving the purification of solvents, distillation of petroleum products, or isolation of essential oils from plant materials. Density differences, viscosity differences, and solubility differences play roles in other separation techniques but are not the primary factors in fractional distillation. These methods have their own applications, but they do not provide the specific selectivity achieved through the differential boiling points of the components in a liquid mixture.