Question

Fractional distillation is used to separate liquids which differ in their boiling points by: (a) \(5^{\circ} \mathrm{C}\) (b) \(10^{\circ} \mathrm{C}\) (c) \(30^{\circ} \mathrm{C}-80^{\circ} \mathrm{C}\) (d) Less than \(20^{\circ} \mathrm{C}\)

Short answer

Fractional distillation is used when boiling points differ by less than 20°C; answer is (d).

Step-by-step solution

Understand Fractional Distillation

Fractional distillation is a technique used to separate mixtures of liquids that have different boiling points. It is effective when the boiling points of the components differ by a small margin because it allows for repeated vaporization and condensation, improving the separation of closely boiling liquids.

Consider Boiling Points Difference

For fractional distillation to be necessary, the difference in boiling points needs to be small enough that simple distillation is not effective. Typically, fractional distillation is used for separating mixtures where the boiling points differ by less than 25°C.

Evaluate Given Options

Review the given options and identify which ones present a boiling point difference small enough to require fractional distillation. This method is most applicable when the boiling point differences are less than 20°C, which is mentioned in option (d).

Conclusion

Based on our evaluation, fractional distillation is most suitable for mixtures where the difference in boiling points is less than 20°C, as stated in option (d).

Key concepts

Boiling Points

In the process of distillation, understanding the boiling point of each component is crucial. The boiling point is the temperature at which a liquid turns into vapor. Each substance has a unique boiling point, which is influenced by intermolecular forces. The stronger these forces are, the higher the boiling point of the substance.

In a mixture, the component with the lower boiling point will vaporize first. Fractional distillation capitalizes on this concept by allowing components to separate based on their boiling points. This inherent property of substances helps guide the separation process, ensuring a more precise separation of components with closely related boiling points.

When deciding between simple and fractional distillation, the boiling point difference plays a critical role. If the difference is large, typically greater than 25°C, simple distillation might suffice. However, for a boiling point difference that is less than 25°C, fractional distillation becomes more efficient, as it can better discern between closely related components.

Separation of Mixtures

Separating mixtures into their individual components is a fundamental aspect of chemistry. Fractional distillation is a specialized technique aimed at doing just that, especially for liquid mixtures that have similar boiling points. This method utilizes repeated cycles of vaporization and condensation to increase the purity of each separated component.

When a mixture contains liquids with boiling points that differ slightly, their vapors can easily mix. Fractional distillation employs a fractionating column. This column provides a surface for multiple vaporization-condensation cycles which improves the separation. As vapor rises and condenses, it becomes enriched with the lower boiling point component, eventually allowing the liquids to be collected separately at the top of the column.

While simple distillation might struggle to separate such mixtures, fractional distillation enhances separation efficacy.

• Enriched component vapors at the top.
• Improved purity with each cycle.
• Ideal for small boiling point differences.

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Vaporization and Condensation

To understand fractional distillation's effectiveness, one must grasp the concepts of vaporization and condensation. Vaporization is the process where liquid turns into vapor as it gains energy, while condensation is the reverse, where vapor loses energy and turns back into liquid.

In fractional distillation, the mixture is heated, causing the component with the lowest boiling point to vaporize first. As the vapor rises through the fractionating column, it cools and condenses on surfaces within the column. This cycle of vaporization and condensation occurs repeatedly as the column is designed to support multiple temperature zones, each refining the separation.

Every time a vapor condenses and revaporizes, it becomes richer in one component - usually the one with the lower boiling point. This sequence continues until the desired purity is reached, allowing distinct separation of components

• Heats mixture to initiate vaporization.
• Column surfaces for condensation cycles.
• Lowers vapor pressure facilitates separation.