Question

Many heat-sensitive and oxygen-sensitive solids, such as camphor, are purified by warming under vacuum. The solid vaporizes directly, and the vapor crystallizes on a cool surface. What phase changes are involved in this method?

Short answer

Sublimation and deposition are the phase changes.

Step-by-step solution

- Identify Initial Phase

Start by recognizing the initial state of camphor, which is a solid.

- Understand Sublimation

When camphor is warmed under vacuum, it vaporizes directly from the solid phase to the gas phase. This phase change is called sublimation.

- Cooling and Crystallization

The vapor then moves to a cool surface, where it crystallizes back into the solid phase. This phase change is called deposition.

- Summarize Phase Changes

The process involves two key phase changes: sublimation (solid to gas) and deposition (gas to solid).

Key concepts

Sublimation

Sublimation is a phase change where a solid transforms directly into a gas without passing through the liquid phase. This happens under specific conditions, such as low pressure or high temperature.

Examples of substances that undergo sublimation include:

• Camphor
• Dry ice
• Solid iodine

During sublimation, the molecules in the solid gain enough energy to break free from their fixed positions. This allows them to move freely as a gas. It works well for substances with weak intermolecular forces and helps in several industrial and scientific applications.

For instance, sublimation is used in:
• Portable air fresheners
• Freeze-drying foods
• Purifying heat-sensitive materials

Deposition

Deposition is the process where a gas changes directly into a solid without becoming a liquid first. It is the opposite of sublimation and happens when the gas loses enough energy to allow the molecules to settle into a fixed position as a solid.

Common examples of deposition include:

• Frost formation
• Soot formation

Deposition is essential in different contexts, such as:

• Coating surfaces with thin layers of material in technological and industrial applications, like semiconductor fabrication.
• Natural phenomena like snow formation directly from the water vapor in suitable conditions.

It ensures that substances can be collected in their solid form after being vaporized, as seen in purification techniques.

Purification Techniques

Purification techniques are methods used to separate and remove impurities from a substance to obtain a pure product. For sensitive solids like camphor, the process involves controlled heating and cooling to utilize phase changes effectively.

Here are the steps involved in using sublimation and deposition for purification:

• Heat the solid under vacuum so it vaporizes (sublimation).
• Direct the vapor to a cool surface where it condenses and crystallizes (deposition).

This method benefits from the fact that impurities often do not sublimate with the solid and remain behind.

Other examples of purification techniques include:

• Distillation
• Filtration
• Crystallization

Each technique is chosen based on the physical and chemical properties of the substance and the impurities present. Sublimation and deposition provide a clean and effective way to purify sensitive materials without degrading them.