Deposition is the reverse of sublimation and is an important phase change in IB Chemistry Topic 1 (Stoichiometry) and Topic 5 (Energetics). It describes how a substance transforms directly from a gas into a solid without becoming a liquid in between. Deposition helps explain frost formation, thin-film manufacturing, and the behavior of substances at low temperatures and pressures. Understanding deposition also reinforces key ideas about phase diagrams, vapor pressure, and intermolecular forces.
What Is Deposition?
Deposition is the phase change where a gas transforms directly into a solid without passing through the liquid state.
This transformation happens when:
- Gas particles lose energy rapidly
- Intermolecular forces pull particles into a solid lattice
- Temperature and pressure conditions fall below the triple point
Deposition is an exothermic process because it releases energy as particles lock into a solid structure.
Why Deposition Occurs
For deposition to take place, two conditions must be met:
1. Gas particles must lose enough kinetic energy
When the temperature drops, gas particles slow down.
If they lose sufficient energy, they can no longer remain separate and begin to clump together.
2. Intermolecular forces must be strong enough to pull particles into a solid
Once a particle slows, intermolecular forces (such as London dispersion forces or hydrogen bonding) draw it into a lattice.
3. Conditions must fall below the triple point
If pressure is low enough, a substance cannot exist as a liquid.
In these cases, gas transitions directly into a solid.
Examples of Deposition
1. Frost formation
Water vapor in the air deposits as ice crystals on cold surfaces.
No liquid water is formed.
2. Iodine crystals forming from vapor
Iodine vapor cools and forms shiny purple-black crystals.
3. Soot deposition
Carbon particles in hot gas streams solidify on cooler surfaces.
4. Snow formation in clouds
Water vapor deposits onto dust particles to form snowflakes.
5. Thin-film and semiconductor production
Specialized deposition techniques create solid coatings from vaporized materials.
These examples show deposition in both natural and industrial settings.
Deposition in Terms of Particle Behavior
According to kinetic theory:
- Gas particles move rapidly
- When cooled, their movement slows
- Reduced kinetic energy allows intermolecular attractions to dominate
- Particles lock into solid positions
Since particles stick together and release energy, deposition is exothermic.
Why Deposition Is Exothermic
Just as melting and vaporization are endothermic, the reverse processes (freezing, condensation, and deposition) release energy.
During deposition:
- Gas particles form strong intermolecular attractions
- Energy is released as they settle into a more ordered structure
- This energy is transferred to the surroundings
This is why frost can appear even when the temperature is only slightly below freezing—heat is released when vapor becomes solid.
Deposition on a Phase Diagram
A phase diagram shows deposition as the line between:
- The solid phase
- The gas phase
Below the triple point, liquids cannot exist.
Any gas cooled under these conditions will deposit into a solid.
For carbon dioxide, deposition occurs at normal atmospheric pressure because CO₂’s triple point is above 1 atm. This is why dry ice forms from gaseous CO₂ under the right cooling conditions.
Real-World Applications of Deposition
1. Thin-film deposition in electronics
Used to create semiconductor layers, microchips, and solar cells.
2. Freeze-drying
Food or pharmaceuticals are frozen, and the ice is removed by sublimation and redeposited on cold surfaces.
3. Atmospheric science
Deposition is essential in cloud formation, frost patterns, and snowflake crystallization.
4. Environmental chemistry
Pollutants like soot or sulfur compounds can deposit onto buildings and surfaces.
Common IB Misunderstandings
“Deposition requires energy input.”
Incorrect—deposition releases energy.
“Deposition is rare.”
Not true—frost and snow formation occur frequently.
“Deposition and freezing are the same.”
Freezing is liquid → solid.
Deposition is gas → solid.
“Only water undergoes deposition.”
Many substances deposit under the right conditions.
FAQs
Why does frost form without water droplets?
Because water vapor deposits directly as ice when surfaces are cold enough.
Is deposition reversible?
Yes—the reverse is sublimation (solid → gas).
Can deposition occur at room pressure?
Yes, if temperature is low enough and vapor pressure conditions are right.
Conclusion
Deposition is the direct phase change from gas to solid and occurs when particles lose energy rapidly and intermolecular forces draw them into a solid lattice. As an exothermic process, deposition releases heat, and it plays a major role in natural phenomena like frost and snow as well as industrial thin-film production. Understanding deposition helps IB Chemistry students interpret phase diagrams, analyze energy changes, and explain particle behavior at different temperatures.
