Deposition
Summary
Deposition is the phase change in which a gas becomes a solid directly, skipping the liquid phase. It is the reverse of sublimation, so it is exothermic and lowers entropy. On a phase diagram it is the gas-to-solid crossing of the solid–gas boundary, which lies below the triple-point pressure.
Detail
Meaning: Deposition is the change from gas to solid, such as water vapor forming frost on a cold surface. Gas molecules lose kinetic energy and lock into a lattice, releasing heat to the surroundings. So ΔH is negative and entropy decreases (ΔS < 0). It is the exact reverse of sublimation.
Equation: ΔHdep = −ΔHsub, where both are enthalpy changes in J/mol. Because ΔHsub ≈ ΔHfus + ΔHvap at the same temperature, deposition releases about as much heat as condensation plus freezing combined.
On the MCAT: On a phase diagram, deposition is the move from the gas region to the solid region across the solid–gas line. That line is the one reached at pressures below the triple point. Cooling a gas at constant pressure below the triple-point pressure crosses it. On a cooling curve, temperature stays flat during the change while heat leaves the system. Questions ask for the signs of ΔH, ΔS and ΔG. ΔG = ΔH − TΔS is negative at low enough temperature, so deposition is favored when it is cold.
Common confusion: Students swap deposition with sublimation or condensation. Deposition is gas → solid and releases heat. Sublimation is solid → gas and absorbs heat. Condensation is gas → liquid. A quick check: any change that goes toward the more ordered phase is exothermic and has ΔS < 0.
Sources
- Chemistry: The Central Science
- Chemistry: A Molecular Approach
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.