Sublimation is a key phase change discussed in IB Chemistry Topic 1 (Stoichiometry) and Topic 5 (Energetics). It describes the direct transition of a solid into a gas without passing through the liquid state. This unusual process helps explain the behavior of certain molecular solids, volatile substances, and materials used in laboratory techniques. Understanding sublimation provides insight into particle motion, intermolecular forces, and phase diagrams.
What Is Sublimation?
Sublimation is the phase change in which a solid transforms directly into a gas without becoming a liquid.
This happens when:
- The solid particles gain enough energy to break free from their fixed positions
- Intermolecular forces are weak enough for particles to escape directly into the gas phase
- Vapor pressure of the solid exceeds atmospheric pressure at certain temperatures
Sublimation is an endothermic process because energy is absorbed to overcome intermolecular forces.
Examples of Substances That Sublime
Only certain solids undergo sublimation under normal conditions.
Common examples include:
1. Dry ice (solid CO₂)
Sublimes at –78°C, producing fog-like gas.
2. Iodine
Forms purple vapor when gently heated.
3. Ammonium chloride
Breaks into NH₃ and HCl gases, which recombine on cooler surfaces.
4. Naphthalene
Found in mothballs; slowly sublimates at room temperature.
5. Solid air fresheners
Rely on slow sublimation to release fragrance.
These substances have relatively weak intermolecular forces or sufficiently high vapor pressures.
Why Sublimation Occurs
Sublimation is determined by:
- Molecular structure
- Intermolecular forces
- Vapor pressure
- Temperature and pressure conditions
If the vapor pressure of a solid becomes high enough, it can exceed atmospheric pressure before the solid melts. When that happens, the solid transitions directly into a gas.
Weak intermolecular forces (like London dispersion forces) make sublimation easier.
Sublimation in Terms of Kinetic Theory
According to kinetic theory:
- Particles in a solid vibrate
- With enough energy, some can escape directly into the gas phase
- Temperature increases the energy and frequency of escape
- This creates a continuous sublimation process
Even at room temperature, some solids slowly sublime if their vapor pressure is high enough.
Sublimation Is an Endothermic Process
Just like vaporization and melting, sublimation requires energy input.
The required energy is called:
Enthalpy of sublimation (ΔHsub)
This energy:
- Breaks intermolecular forces
- Increases particle separation
- Increases kinetic energy
This is why dry ice feels extremely cold—it absorbs heat from the surroundings rapidly.
Sublimation on Phase Diagrams
On a phase diagram, the sublimation line separates:
- The solid region
- The gas region
Below the triple point, the solid cannot become a liquid because the pressure is too low.
Therefore, substances below their triple point only sublime.
Carbon dioxide at atmospheric pressure is a perfect example.
Uses of Sublimation in Real Life
Sublimation has many practical applications:
1. Freeze-drying (lyophilization)
Used to preserve food and pharmaceuticals by sublimating ice directly from frozen samples.
2. Purification of solids
Iodine and naphthalene are purified by sublimation.
3. Fog and smoke effects
Dry ice sublimation creates dense clouds in stage effects.
4. Air fresheners
Solid fragrance blocks slowly sublime to release scent.
5. Manufacturing electronics
Sublimation helps deposit thin films of materials onto surfaces.
How IB Chemistry Tests Sublimation
You may be asked to:
- Identify a phase change from a heating curve
- Explain why sublimation is endothermic
- Describe vapor pressure conditions
- Interpret phase diagrams and triple points
- Distinguish sublimation from evaporation or boiling
These questions often appear in Paper 2 and multiple-choice form in Paper 1.
Common IB Misunderstandings
“Sublimation only happens at extremely high temperatures.”
Not true—dry ice sublimes at –78°C and iodine at modest heating.
“Sublimation and evaporation are the same.”
Evaporation is liquid → gas; sublimation is solid → gas.
“All solids can sublime.”
Only those with high vapor pressure and weak intermolecular forces do.
“Sublimation does not require energy.”
False—sublimation is strongly endothermic.
FAQs
Why does dry ice sublime instead of melt?
Because at atmospheric pressure, CO₂’s triple point is above 1 atm. It cannot exist as a liquid under normal conditions.
Can sublimation happen at room temperature?
Yes—iodine, naphthalene, and air fresheners can sublime slowly at room temperature.
Is deposition the reverse of sublimation?
Yes—gas → solid is deposition.
Conclusion
Sublimation is the direct transformation of a solid into a gas, bypassing the liquid phase. It occurs when solids have high vapor pressure or weak intermolecular forces and is an endothermic process requiring energy input. Sublimation is essential in food preservation, purification, atmospheric chemistry, and phase transition analysis in IB Chemistry.
