Short Answer

Sublimation on the phase diagram refers to the direct change of a substance from the solid state to the gas state without passing through the liquid state. On a phase diagram, sublimation occurs along the boundary line that separates the solid and gas regions.

This line is called the sublimation curve. At any temperature and pressure lying on this line, both solid and gas exist in equilibrium. Substances like dry ice (solid CO₂) commonly undergo sublimation because they do not have a stable liquid phase under normal pressure.

Detailed Explanation

Sublimation on the Phase Diagram

Sublimation is the process in which a solid changes directly into a gas without first becoming a liquid. On a phase diagram, this process is clearly shown by a specific curve known as the sublimation line or solid–gas boundary. This line separates the solid region from the gas region and represents the temperature and pressure conditions at which solid and gas can exist together in equilibrium.

Understanding sublimation on a phase diagram helps us explain how certain substances behave in daily life and in scientific applications. For example, dry ice disappears without melting, and snow can slowly vanish even when the temperature is below 0°C. These observations are directly connected to the sublimation line on the phase diagram.

1. Meaning of Sublimation on the Phase Diagram

On the phase diagram, sublimation is shown by the line that divides the solid phase from the gas phase. This line indicates:

• The temperature at which a solid starts to turn into a gas
• The pressure below which the liquid state cannot exist
• Conditions where solid and gas remain in equilibrium

If conditions lie on this line, the substance can undergo sublimation or its reverse process, deposition (gas changing directly into solid).

Many substances have sublimation lines that lie at low pressures, which is why sublimation is commonly observed under low-pressure conditions.

2. Sublimation Curve and Its Importance

The sublimation curve is the boundary between solid and gas. It shows how the sublimation temperature changes with pressure.

Key points about the sublimation curve:

• At low pressures, the curve slopes upward and to the right on a phase diagram.
• Increasing pressure usually prevents sublimation because the solid is forced to melt instead.
• At the triple point, the sublimation curve ends because the substance can also exist as a liquid at that condition.

This curve helps scientists and engineers understand how solids behave in vacuum or low atmospheric pressure environments.

3. Relationship Between Sublimation and the Triple Point

The triple point is the unique condition where solid, liquid, and gas coexist. The sublimation curve ends at the triple point. This means:

• Below the triple point pressure, the liquid state cannot exist.
• Any heating of the solid below this pressure results in sublimation rather than melting.

For example:

• Carbon dioxide at 1 atm does not melt.
• Instead, it jumps directly from solid dry ice to CO₂ gas because its triple point pressure is 5.11 atm.

Thus, sublimation is directly controlled by the location of the triple point on the phase diagram.

4. Examples of Sublimation Explained Through Phase Diagrams

Dry Ice (Solid CO₂)

Dry ice sublimates at normal atmospheric pressure because the pressure is below CO₂’s triple point. This is why:

• Dry ice forms fog instead of melting
• It disappears without leaving liquid behind

Iodine

Iodine crystals can sublime when gently heated, forming a purple vapour.
This behaviour is explained by its sublimation line at moderate pressures.

Snow and Ice

Snow can slowly disappear on cold sunny days even when the temperature is below freezing because sunlight provides enough energy for sublimation.

Camphor and Naphthalene

These substances slowly evaporate from solid form because their sublimation lines lie close to normal pressure conditions.

Phase diagrams explain why some solids easily sublime while others require very special conditions.

5. Factors That Influence Sublimation

Several factors affect sublimation behaviour:

1. Pressure

Low pressure encourages sublimation because liquids become unstable.
If pressure is below the triple point pressure, the solid cannot melt.

1. Temperature

Higher temperature provides energy for molecules to escape directly from solid to gas.

1. Intermolecular Forces

Weak intermolecular forces allow molecules to leave the solid phase more easily.
This is why molecular solids like I₂ and naphthalene sublime more readily than ionic or metallic solids.

1. Surface Area

Larger surface area increases sublimation rate, such as powdered ice subliming faster than large ice blocks.

6. Applications of Sublimation

Sublimation is used in many technologies:

• Freeze-drying (lyophilization) to preserve food and medicines
• Manufacture of dry ice for refrigeration
• Purification of solids like camphor or iodine
• Fabrication of thin films in electronics

Understanding the sublimation curve on the phase diagram helps control these processes.

Conclusion

Sublimation on a phase diagram is represented by the solid–gas boundary line, known as the sublimation curve. This line shows the temperature and pressure at which solid and gas phases coexist in equilibrium. Below the triple point pressure, sublimation becomes the only possible phase transition from solid to gas. Knowing how sublimation appears on a phase diagram helps explain natural phenomena and supports important industrial and scientific applications.