The Difference Between Deposition & Sublimation

Though often only liquids are thought to evaporate, solids undergo evaporation too!

Sublimation is the process by which molecules go directly from solid into the vapor or gas phase.

Deposition is the process by which molecules go directly from the gas phase into the solid phase. Deposition chemistry occurs when molecules settle out of the gas phase and into the solid phase.

Sublimation and deposition chemistry are examples of phase changes. You've probably seen a phase change before. When you boil water to make mac and cheese the water starts to steam. This is evaporation, or the liquid phase of water going to the vapor phase of water.

Solid CO₂, which is sometimes called dry ice, goes straight from solid to gas at room temperature. This is another example of a phase change, specifically: sublimation.

Take a look at the boiling water for mac and cheese example again. You add energy to the liquid water in order to get it to change phase. When you do this, the water goes from a more ordered phase (liquid) to a less ordered phase (vapor). Thus, energy is required whenever the phase changes to a less ordered state.

Why is this? The attractive forces that hold molecules together must be overcome. The only way to do this is to add energy until the molecules no longer stay together.

This means that melting, vaporization and sublimation are all endothermic processes. They require the addition of energy or heat.

The reverse processes (freezing, condensation and deposition) are all exothermic processes. This means that they release heat. You've probably experienced this if you get too close to steam. The steam is hot because when you touch it, it immediately condenses and releases heat!

Molecules in a solid are much more tightly held together than they are in a liquid. For this reason the vapor pressure of a solid (yes, solids have a vapor pressure too!) is lower than it is for the liquid.

The molar heat of sublimation is the energy required to sublime one mole of a solid. It is the sum of the molar heats of fusion and vaporization. The molar heat of fusion is the energy required to melt one mole of a solid, while the molar heat of vaporization is the energy required to vaporize one mole of a liquid:

\(\Delta H_{sublimation} = \Delta H_{fusion}+\Delta H_{vaporization}\)

Phase diagrams give the relationship among the solid, liquid and vapor phase of a compound. It summarizes the conditions at which the substance exists as a solid, liquid or gas.

For example, the curve between the solid and vapor phase shows how vapor pressure changes with temperature. The point at which all three curves meet is called the triple point. This is the only condition where all three phases can be in equilibrium with each other.

Phase diagrams are helpful in predicting how a substance will behave given a certain temperature and pressure.

The most well-known example of sublimation is dry ice. Of course, dry ice is not actually ice, it is frozen CO₂. The CO₂ sublimates at room temperature.

Other examples include air fresheners. The solid in an air freshener can sublimate in order to make the room smell nice. Naphthalene, which is used to make moth balls, is another example of odorous sublimation. It sublimates quickly, and the smell keeps the moths away.

Another example is medication. Often medication bottles will say that you should keep them in a cool place or at room temperature. This is because there is a possibility that some of the components involved could sublimate if they get heated up too much.

One example of deposition of a gas to solid that you may be familiar with is the formation of frost. When it's below zero outside it becomes so cold that any water vapor goes straight from the vapor phase into solid phase. That's why you see frost first thing in the morning!