Sublimation and Deposition
Sublimation is the direct change of a solid into a gas without passing through the liquid state, and deposition is the direct change of a gas into a solid. In brief: dry ice (solid carbon dioxide) sublimes at −78.5 °C at normal pressure instead of melting, and frost on a cold window forms by deposition of water vapor as ice crystals.
Both changes skip the liquid stage and are physical changes. They explain why frozen laundry dries outdoors in winter, why mothballs slowly disappear and why freeze-dried food keeps for so long. This article explains the two processes, compares them with the other changes of state, shows a worked energy example and lists their uses.
What is sublimation?
In sublimation, particles at the surface of a solid gain enough energy to escape directly into the gas phase. The process is endothermic, so it absorbs heat from the surroundings. Because the heat is taken from the environment, dry ice feels extremely cold and cools the objects near it.
Sublimation does not require the solid to be at a special temperature, and a small amount of sublimation occurs even in ordinary conditions. For instance, snow can disappear on a sunny, dry, windy day without melting into water.
What is deposition?
Deposition is the opposite process: gas particles lose energy and settle directly into a solid structure. It releases heat. Frost, hoarfrost on grass and the delicate patterns of ice on a window are all formed by deposition of water vapor when the surface is below 0 °C.
Snowflakes also grow by deposition, as water vapor in cold clouds turns directly into ice on the surface of existing ice crystals.
Why do some solids sublime?
Whether a solid sublimes or melts depends on temperature and pressure. Each substance has a triple point, the unique temperature and pressure at which solid, liquid and gas coexist. Below the triple-point pressure, a solid cannot exist in equilibrium with a liquid, so heating it produces gas directly.
For carbon dioxide, the triple-point pressure is about 5.2 times atmospheric pressure. At normal air pressure the liquid cannot exist, so dry ice always sublimes. Iodine and naphthalene also sublime readily because their particles are held together by weak attractions.
All six changes of state
| Change | From | To | Energy |
|---|---|---|---|
| Melting | Solid | Liquid | Absorbed |
| Freezing | Liquid | Solid | Released |
| Evaporation / boiling | Liquid | Gas | Absorbed |
| Condensation | Gas | Liquid | Released |
| Sublimation | Solid | Gas | Absorbed |
| Deposition | Gas | Solid | Released |
Common examples
| Process | Example | What you observe |
|---|---|---|
| Sublimation | Dry ice | Solid shrinks and releases cold gas |
| Sublimation | Mothballs | Slowly disappear, strong smell |
| Sublimation | Snow on a dry, windy day | Snow vanishes without wet ground |
| Sublimation | Iodine crystals warmed gently | Violet vapor forms |
| Deposition | Frost on a window | Ice crystals grow in patterns |
| Deposition | Iodine vapor on a cold surface | Dark crystals form |
Worked example: energy for sublimation
The specific latent heat of sublimation of carbon dioxide is about 571 J/g at its sublimation temperature of −78.5 °C.
- Sublimation of 100 g of dry ice. Q = m × L = 100 g × 571 J/g = 57,100 J, or about 57.1 kJ.
- Cooling effect. The heat is taken from the surroundings. If this heat came from 500 g of water with specific heat capacity 4.18 J/(g·°C), the water would cool by 57,100 ÷ (500 × 4.18) = 27.3 °C, if no other heat entered.
- Volume of gas formed. The molar mass of CO2 is 44 g/mol, so 100 g is 100 ÷ 44 = 2.27 mol. At 0 °C and 1 atm, a mole of gas occupies about 22.4 L, so the gas volume is about 2.27 × 22.4 = 50.9 L.
A small block of solid therefore turns into a very large volume of gas. This is why dry ice must never be sealed in a closed container, because the rising pressure can cause an explosion.
Where are sublimation and deposition used?
- Cooling: dry ice keeps food and medical samples cold without leaving liquid.
- Freeze-drying: frozen food is placed in a vacuum so that ice sublimes, leaving a light, dry product.
- Purification: a substance that sublimes can be separated from one that does not, then recovered by deposition on a cold surface.
- Electronics: thin films of material are produced by vapor deposition on a surface.
How were these changes discovered?
The word sublimation was used by alchemists for the purification of solids by heating them into vapor and collecting the crystals. The modern understanding came with phase diagrams in the 1800s, which showed how states depend on temperature and pressure. Solid carbon dioxide was first produced in the 1830s, and dry ice became an industrial product in the 1920s.
Which mistakes are common?
Mistake 1: Thinking that the fog from dry ice is carbon dioxide gas
Carbon dioxide gas is invisible. The white fog is made of tiny water droplets condensed from the air by the cold gas.
Mistake 2: Saying that all solids sublime at normal pressure
Most solids melt first. Only some, such as dry ice, iodine and naphthalene, sublime easily.
Mistake 3: Confusing deposition with condensation
Condensation forms a liquid from a gas. Deposition forms a solid directly from a gas.
Mistake 4: Touching dry ice with bare hands
At −78.5 °C, dry ice can cause frostbite. Always use insulated gloves or tongs.