Thermochemical statement
The phrase is heat being used in sublimation has a clear thermochemistry answer: sublimation (solid \(\rightarrow\) gas) requires energy input. Heat is absorbed by the substance as it transitions directly from the solid phase to the gas phase, so the heat flow into the system is positive under the usual sign convention.
Physical basis for heat absorption
A solid has particles held in place by intermolecular attractions (and, for some solids, extended lattice interactions). A gas has particles that are far apart with much weaker attractive interactions. The solid-to-gas transition therefore increases the potential energy associated with separating particles. That energy increase must come from heat absorbed from the surroundings (or from an energy source supplying heat to the sample).
- Microscopic view: increased average separation of particles, weaker net attractions, higher potential energy.
- Macroscopic view: heat flows into the sample during sublimation, often producing cooling of the surroundings.
Enthalpy of sublimation and heat calculations
At constant pressure, the heat absorbed by a phase change is commonly modeled with an enthalpy change. For sublimation:
\[ q_p = m \cdot \Delta H_\text{sub} \]
where \(q_p\) is the heat absorbed at constant pressure, \(m\) is the amount of substance (in grams) when \(\Delta H_\text{sub}\) is expressed in \(\mathrm{J/g}\), and \(\Delta H_\text{sub}\) is the enthalpy of sublimation. When \(\Delta H_\text{sub}\) is tabulated per mole, an equivalent expression is \(q_p = n \cdot \Delta H_\text{sub}\).
Relationship to fusion and vaporization
Sublimation can be treated as a two-stage path (solid \(\rightarrow\) liquid \(\rightarrow\) gas) for enthalpy accounting. Because enthalpy is a state function, Hess’ law gives:
\[ \Delta H_\text{sub} = \Delta H_\text{fus} + \Delta H_\text{vap}. \]
This identity emphasizes that sublimation contains both the energetic cost of disrupting the solid structure (fusion-like contribution) and the energetic cost of separating molecules into the gas phase (vaporization-like contribution).
Contrast with deposition
Deposition (gas \(\rightarrow\) solid) is the reverse of sublimation. Its enthalpy change has the opposite sign: \(\Delta H_\text{dep} = -\Delta H_\text{sub}\). Heat is released to the surroundings in deposition.
Phase-change enthalpy signs
| Phase change | Direction | Typical sign of \(\Delta H\) | Heat flow for the substance |
|---|---|---|---|
| Melting (fusion) | solid \(\rightarrow\) liquid | \(\Delta H_\text{fus} > 0\) | absorbed (\(q > 0\)) |
| Vaporization | liquid \(\rightarrow\) gas | \(\Delta H_\text{vap} > 0\) | absorbed (\(q > 0\)) |
| Sublimation | solid \(\rightarrow\) gas | \(\Delta H_\text{sub} > 0\) | absorbed (\(q > 0\)) |
| Freezing | liquid \(\rightarrow\) solid | \(\Delta H_\text{frz} = -\Delta H_\text{fus}\) | released (\(q < 0\)) |
| Condensation | gas \(\rightarrow\) liquid | \(\Delta H_\text{cond} = -\Delta H_\text{vap}\) | released (\(q < 0\)) |
| Deposition | gas \(\rightarrow\) solid | \(\Delta H_\text{dep} = -\Delta H_\text{sub}\) | released (\(q < 0\)) |
Visualization: phase diagram location of sublimation
Everyday examples and energy source
Dry ice (\(\mathrm{CO_2(s)}\)) sublimates at 1 atm and absorbs heat from the surrounding air and nearby surfaces; the surroundings can feel colder because they supply the required energy. Iodine crystals can also produce visible vapor by sublimation when warmed, again reflecting heat absorption by the solid during the phase change.
Common confusion with temperature
Heat absorption during sublimation does not require a high temperature, and it does not guarantee that the sample temperature rises. Phase changes can absorb energy at nearly constant temperature while the latent heat is consumed by the transition rather than by raising kinetic energy. The central thermochemical statement remains: energy flows into the substance during sublimation.