Understanding Phase Transitions

Vaporization: Evaporation and Boiling

To produce vaporization, liquid particles must reach sufficient kinetic energy to overcome the forces holding them together. This allows them to separate and move independently. Vaporization can occur in two ways: evaporation and boiling.

Evaporation occurs at all temperatures but only on the surface of the liquid. It happens more rapidly when temperature increases or pressure decreases. Not all liquids evaporate at the same rate; those that evaporate quickly are called volatile.

Boiling occurs throughout the volume of the liquid but only at a specific temperature called the boiling point. The boiling point is characteristic of each pure substance but depends on pressure (it increases as pressure increases).

Condensation or Liquefaction

In a gas, particles are far apart and move independently; that is, the movement of a particle does not depend on other particles except when they collide. When two particles collide, they bounce (similar to billiard balls).

If we remove heat from a gas, the particles move more slowly and have less kinetic energy, thus lowering the gas temperature. As we continue removing heat, its temperature will continue to decline until the particles have so little kinetic energy that when they collide, the attractive forces prevent them from bouncing and hold them together (while still moving, sliding past each other). These particles have transitioned to the liquid state.

The particles that transition to the liquid state first will be those with slightly less kinetic energy than the average in the gaseous state. However, as we remove heat from the gas, the average kinetic energy of the remaining particles in the gaseous state stays constant, and therefore the gas temperature remains constant. This temperature is characteristic of each pure substance and coincides with the temperature at which that substance boils, i.e., the boiling point.

Sublimation

Sublimation, or direct sublimation (also called progressive sublimation), is similar to melting, with the difference that when particles overcome the cohesive forces holding them in fixed positions (forming a network or lattice), instead of starting to move by sliding over each other (but staying together), they begin to move independently. That is, they go directly to a gaseous state rather than the liquid state.

Although all substances can sublimate, in most cases, this process does not occur under conditions we are used to seeing. For this process to occur, pressure and temperature conditions must be different from regular conditions. Only a few substances, such as carbon dioxide (dry ice) and naphthalene, sublime at normal atmospheric pressure. For most substances, sublimation occurs only at very low pressures. (See the section on phase diagrams below).