Problem 34 - A Complete Resource Book in Chemistry for JEE Main

Question

Liquids which decompose below their normal boiling points can be distilled at lower temperature by (a) decreasing the pressure (b) heating in sand bath (c) increasing the pressure (d) heating in water bath

Step-by-Step Solution

Answer

Decreasing the pressure (option a) allows distillation at a lower temperature.

Step 1: Understand the Problem

We need to determine how to distill a liquid that decomposes below its normal boiling point. Distillation is the process of separating components in a mixture based on differences in boiling points.

Step 2: Identify the Distillation Principle

Liquids boil when their vapor pressure equals the surrounding pressure. If we reduce the external pressure, the boiling point will also decrease, allowing us to distill at a lower temperature.

Step 3: Evaluate the Options

Examine each option: - (a) Decreasing the pressure lowers the boiling point. - (b) Heating in a sand bath affects heating method but not pressure. - (c) Increasing the pressure would increase the boiling point. - (d) Heating in a water bath affects heating method but not pressure.

Step 4: Select the Correct Option

Based on the principle that reducing pressure lowers boiling points, option (a) decreasing the pressure, is the correct solution as it allows distillation at a temperature below the normal boiling point without causing decomposition.

Key Concepts

Boiling Point

The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure surrounding it. At this point, the liquid transforms into a gas, a process we refer to as boiling.
This is a critical concept in distillation, a common method for separating mixtures.
Boiling points can vary based on:

• External pressure: Lower external pressure leads to a lower boiling point, while higher external pressure results in a higher boiling point.
• The nature of the liquid: Different liquids have unique boiling points due to their distinct intermolecular forces.

In a practical sense, understanding boiling points is essential in scenarios where certain liquids decompose if exposed to their normal boiling temperatures. By manipulating the external factors, it's possible to adjust the boiling temperature, which is particularly useful when trying to avoid decomposition during processes like distillation.

Vapor Pressure

Vapor pressure is a crucial concept that explains why boiling occurs. It is defined as the pressure exerted by a vapor in equilibrium with its liquid or solid phase.

• When a liquid is enclosed in a container, its molecules escape into the gas phase, creating pressure. This is vapor pressure.
• As the temperature increases, more molecules have the energy to escape, increasing the vapor pressure of the liquid.

It's important to note that boiling happens when a liquid's vapor pressure equals the external or atmospheric pressure. If the vapor pressure is equal to or greater than the surrounding pressure, the liquid will transition to the gas phase even at lower temperatures. This understanding is critical when distilling heat-sensitive liquids, as adjusting the external pressure can facilitate boiling at reduced temperatures, preventing decomposition.

Pressure Reduction

Pressure reduction is an effective strategy to lower the boiling point of a liquid. When external pressure is decreased, it enables boiling at lower temperatures.
Here's how it works:

• Reducing pressure decreases the amount of energy required for the molecules to escape the liquid phase.
• This means that the liquid can boil at a temperature lower than its normal boiling point, which is beneficial in distillation processes.

Reducing pressure is particularly useful in scenarios where certain liquids decompose when heated to higher temperatures. By lowering the boiling point through pressure reduction, we can distill these heat-sensitive liquids without triggering decomposition. This approach allows for the efficient separation of components without compromising the integrity of the substances involved.