Question

A mixture contains four solid organic compounds (A), (B), (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

Short answer

The compound (C) can be separated by sublimation.

Step-by-step solution

Understand the Problem Statement

We have a mixture of solid organic compounds (A), (B), (C), and (D). On heating, compound (C) changes directly from the solid state to the vapor state. Our goal is to identify a method to separate (C) from the mixture.

Identify the Process Described

The process where a solid changes directly into a vapor on heating, and then condenses back to a solid on cooling, is called sublimation. This key property of sublimation allows us to separate sublimable substances from non-sublimable substances.

Match the Process to Separation Techniques

We have four options to choose from: (a) Sublimation, (b) Crystallization, (c) Distillation, and (d) Fractional distillation. Sublimation is the only technique where a solid is directly converted into a gas upon heating and then recovered as a solid on cooling. This matches the described behavior of compound (C).

Verify the Best Option

Since compound (C) sublimes, the best separation method is sublimation. The other options typically involve liquid phases or are used when separating components with differing boiling points, not appropriate for compounds directly going from solid to vapor.

Key concepts

Solid-Gas Transformation

When a substance transitions from a solid state directly into a gas without passing through the liquid phase, it is known as sublimation. This solid-gas transformation is not common to all materials. Only specific substances possess the ability to sublime. During sublimation, heat energy causes the molecules in a solid to vibrate intensely, eventually freeing them from their fixed positions. These molecules then move apart rapidly, forming a gas.

Sublimation involves:

• The direct conversion of solid into vapor.
• Releasing the vapor as it cools back into a solid.
• Bypassing the liquid phase entirely.

Re-sublimation can be observed in everyday substances, such as dry ice or iodine. The process is handy in scenarios where one needs to separate components that sublime from those that don't.

Separation Techniques

Separation techniques are crucial in the purification and isolation of various substances. When we talk about separating a mixture, the goal is to isolate components based on their specific properties. Different methods apply to diverse situations, but in this context of separating (C) from the mixture, sublimation is most suited.

Key methods include:

• Sublimation: Best used for solids that can transition directly to vapor.
• Crystallization: Typically for purifying solids from a solution.
• Distillation: Applied when separating liquids with different boiling points.
• Fractional Distillation: A more refined form of distillation for closely boiling liquids.

Choosing the right separation technique is crucial. It depends heavily on the nature of the components in the mixture and the physical properties that differentiate them.

Organic Compounds Separation

Separating organic compounds requires understanding their unique properties. Organic compounds, often containing carbon, hydrogen, and other elements, exhibit a range of physical characteristics. Some might be polar or non-polar, volatile or stable, crystalline or amorphous.

In mixtures like our example, recognizing features such as the ability to sublime is vital. While organic chemistry offers multiple ways to achieve separation, sublimation stands out for solids that vaporize without melting. For compound (C) that sublimes directly from solid to vapor, separation becomes straightforward using:

• Sublimation: Exploiting volatility and sublimation tendencies of certain compounds.

This technique is effective, especially when the compound of interest differs significantly from others in its sublimation ability, as seen with (C). Understanding these properties allows chemists to choose the appropriate method for the best results.