Short Answer

Deposition is the process in which a gas changes directly into a solid without passing through the liquid state. On a phase diagram, deposition occurs along the solid–gas boundary line, which is the reverse of sublimation.

At any point along this line, solid and gas exist together in equilibrium. Deposition is commonly seen in frost formation, where water vapor in the air turns directly into ice on cold surfaces.

Detailed Explanation

Deposition

Deposition is the reverse process of sublimation. It occurs when gas particles lose enough energy to settle directly into the solid phase, bypassing the liquid state. On a phase diagram, this process is represented along the solid–gas boundary line. The line shows the temperature and pressure conditions where the gas and solid can coexist in equilibrium.

Deposition is an important concept in chemistry, physics, and natural processes because it explains how solids form from gases without condensation into a liquid. It is also crucial in industrial processes like purification of substances and freeze-drying.

1. Deposition on the Phase Diagram

On a phase diagram:

• The solid–gas line shows conditions for both sublimation and deposition.
• When moving from gas to solid along this line, the process is deposition.
• The line ends at the triple point, where solid, liquid, and gas coexist.

Deposition occurs below the triple point pressure when the liquid phase cannot exist. This explains why certain substances do not form liquids under normal atmospheric conditions.

2. Examples of Deposition
3. Frost Formation

Water vapor in cold air deposits directly onto cold surfaces as ice.

• No liquid water is formed.
• Occurs on windows, leaves, and grass in winter.

1. Iodine Vapors

Iodine gas deposits as shiny violet crystals when cooled.

• Used in chemical purification.
• Reverse process of sublimation in laboratory.

1. Carbon Dioxide

At pressures below its triple point, CO₂ gas deposits as solid dry ice when cooled.

• Common in refrigeration and transport of perishable goods.

1. Snow Formation

In clouds, water vapor can deposit directly as snowflakes at temperatures below freezing.

These examples show that deposition is common in nature and industry.

3. Factors Affecting Deposition

Several factors determine the rate and possibility of deposition:

1. Temperature

• Gas must be cooled to lose kinetic energy.
• Lower temperature favors deposition.

1. Pressure

• Low pressure below the triple point encourages direct gas-to-solid transition.
• High pressure may cause condensation into liquid first.

1. Surface

• Deposition often requires a surface for gas molecules to settle.
• Smooth and cold surfaces enhance deposition.

1. Nature of Substance

• Substances with weak intermolecular forces can deposit more easily.
• Molecular crystals like iodine and naphthalene sublime and deposit readily.

4. Applications of Deposition

Deposition has several practical uses:

• Freeze-drying (Lyophilization): Removes water from foods and medicines by sublimation and deposition.
• Purification of Solids: Deposition of vapors helps obtain pure crystals in chemical laboratories.
• Industrial Production: Solid CO₂ and other gases are prepared and stored using deposition techniques.
• Meteorology: Frost and snow formation prediction relies on understanding deposition.

These applications show the relevance of deposition in science and daily life.

5. Deposition and Sublimation

Deposition is directly linked to sublimation:

• Sublimation: Solid → Gas
• Deposition: Gas → Solid

Both occur along the same solid–gas line on a phase diagram but in opposite directions. Understanding this relationship helps explain natural phenomena like frost and industrial processes like freeze-drying.

Conclusion

Deposition is the direct transition of a gas into a solid without forming a liquid. On the phase diagram, it occurs along the solid–gas boundary line below the triple point pressure. Deposition is common in nature, seen in frost and snow formation, and is used in various industrial processes such as freeze-drying and chemical purification. Its study helps us understand phase changes, equilibrium, and the behaviour of substances under specific temperature and pressure conditions.