Problem 66 - A Complete Resource Book in Chemistry for JEE Main

Question

A mixture contains four solid organic compounds \((\mathrm{A}),(\mathrm{B}),(\mathrm{C})\) and (D). On heating, only (C) changes from solid to vapour state. The compound (C) can be separated from the mixture by (a) sublimation (b) crystallization (c) distillation (d) fractional distillation

Step-by-Step Solution

Answer

(a) sublimation

Step 1: Understand Sublimation

Sublimation is the process where a solid changes directly into a gas without passing through the liquid state. This process is used to separate substances that are volatile (can easily vaporize) from others that are not.

Step 2: Assess the Volatility of Compound (C)

According to the problem, compound (C) is the only one that changes from a solid to a vapor when heated. This means that (C) is volatile and can sublimate, whereas the other compounds in the mixture do not.

Step 3: Choose the Appropriate Separation Technique

Given that (C) sublimes, the best method to separate it from the mixture is sublimation. Other processes like crystallization, distillation, or fractional distillation are not suitable here because they are designed for different types of changes (liquid-liquid separation or purifying liquids).

Step 4: Conclusion

Since sublimation is the most appropriate method for separating a compound that converts directly from solid to vapor, it should be used to separate compound (C) from the mixture.

Key Concepts

Volatile Compounds

In the context of chemistry, volatile compounds are substances that can easily transform from a solid or liquid state to a gas at relatively low temperatures. This ability to vaporize happens because these compounds have weak intermolecular forces—forces between molecules—allowing them to evaporate readily when enough energy in the form of heat is applied to them.
Volatility is a key factor in determining how certain compounds behave under various conditions. For example, if a solid compound like (C) in a mixture is volatile, it means it can straightforwardly change into a vapor, leaving behind other less volatile substances. This property is vital for specific separation techniques like sublimation, which we will dive into in the next sections. By understanding the volatility of compounds in a mixture, one can decide the most efficient and effective method to separate them.

Solid to Vapor

The term "solid to vapor" describes a phase transition where a compound changes directly from a solid state to a gaseous state, bypassing the liquid phase entirely. This process is known as sublimation and is useful in separating volatile solids from non-volatile substances.
Sublimation occurs under specific conditions, usually involving the application of heat to the solid. The heat increases the kinetic energy of the molecules, reducing intermolecular forces, thus allowing the molecules to escape the solid phase and enter the gaseous phase. An everyday example of solid to vapor transition is dry ice, which sublimates at room temperature, turning directly from solid carbon dioxide to carbon dioxide gas.
In the context of the exercise, compound (C) can undergo sublimation, affirming that it is volatile. This unique trait allows it to change phase directly, and it offers an efficient way to separate it from other less volatile compounds in the mixture.

Mixture Separation Techniques

Separation of mixtures into their individual components is a fundamental task in chemistry. The choice of method depends largely on the physical properties of the components involved. One of the simplest and most efficient separation techniques for mixtures containing volatile and non-volatile substances is sublimation.
This technique leverages the ability of volatile compounds to directly change from solid to vapor upon heating.

• Sublimation: Ideal for removing volatile solids from a mixture. As you heat the mixture, the compound that can sublimate (volatile) turns into vapor, while non-volatile solids remain in the container.

Other common separation techniques include:

• Crystallization: Used for purifying solids that form crystals as they precipitate out of a solution.
• Distillation: Suitable primarily for separating liquid mixtures based on differing boiling points.
• Fractional Distillation: Specifically for separating mixtures of liquids with closer boiling points.

Understanding the physical properties of components in a mixture, like volatility, determines which separation technique will be the most appropriate and efficient.