Fractional distillation method is used for purification of liquids when there is a :

Question

Accepted Answer

Fractional distillation is a technique used to separate mixtures of liquids that have close boiling points. The method is particularly effective when the difference in boiling points is small, typically less than 25°C. This is because fractional distillation involves repeated vaporization and condensation cycles within a fractionating column, which allows for the efficient separation of components with similar boiling points. The column provides a large surface area for multiple condensation and vaporization steps, enhancing the separation of the components based on their boiling points.

1. **Explanation for Option 1:**
   - Fractional distillation is specifically designed for mixtures where the components have a small difference in boiling points, generally less than 25°C. The fractionating column allows for the repeated exchange of heat between the rising vapor and the descending liquid, which helps in achieving a high degree of separation. This makes fractional distillation the ideal method for purifying liquids with closely related boiling points.

2. **Explanation for why other options are incorrect:**
   - **Option 2: No difference in their boiling points:** If there is no difference in boiling points, fractional distillation cannot separate the components because the method relies on differences in boiling points to achieve separation. In such cases, other techniques like azeotropic distillation or the use of separating agents might be required.
   - **Option 3: Large difference (greater than 25°C) in the boiling points of liquid:** When there is a large difference in boiling points, simple distillation is usually sufficient. Simple distillation is more efficient and cost-effective for separating components with significantly different boiling points, as it does not require the complex setup of a fractionating column.
   - **Option 4: Small difference in the melting points:** Fractional distillation is concerned with boiling points, not melting points. The separation of components based on melting points would require different techniques, such as recrystallization or sublimation, depending on the nature of the substances involved.

In conclusion, fractional distillation is best suited for separating liquids with a small difference in their boiling points, which is why Option 1 is the correct answer.