How to Predict Phase Transitions Using a Phase Diagram

By Peter J. Mikulecky, Chris Hren

Each state of matter, whether solid, liquid, or gas, is called a phase. Phase diagrams are useful tools for describing the states of a given type of matter across different temperatures and pressures.

When matter moves from one phase to another because of changes in thermal energy and/or pressure, that matter is said to undergo a phase transition. Moving from liquid to gas is called boiling, and the temperature at which boiling occurs is called the boiling point. Moving from solid to liquid is called melting, and the temperature at which melting occurs is called the melting point. The melting point is the same as the freezing point, but freezing implies matter moving from liquid to solid phase.

At the surface of a liquid, molecules can enter the gas phase more easily than elsewhere within the liquid because the motions of those molecules aren’t as constrained by the molecules around them. So these surface molecules can enter the gas phase at temperatures below the liquid’s characteristic boiling point. This low-temperature phase change is called evaporation and is very sensitive to pressure. Low pressures allow for greater evaporation, while high pressures encourage molecules to re-enter the liquid phase in a process called condensation.

The pressure of the gas over the surface of a liquid is called the vapor pressure. Understandably, liquids with low boiling points tend to have high vapor pressures because the particles are weakly attracted to each other. At the surface of a liquid, weakly interacting particles have a better chance to escape into the vapor phase, thereby increasing the vapor pressure. See how kinetic molecular theory helps make sense of things?

At the right combination of pressure and temperature, matter can move directly from a solid to a gas, or vapor. This type of phase change is called sublimation, and it’s the kind of phase change responsible for the white mist that emanates from dry ice, the common name for solid carbon dioxide. Movement in the opposite direction, from gas directly into solid phase, is called deposition.

The table summarizes the phase changes.

Phase Changes
Phase Change (Increasing Energy) Phase Change (Decreasing Energy)
Melting: Solid to liquid Freezing: Liquid to solid
Vaporization (evaporation or boiling): Liquid to gas Condensation: Gas to liquid
Sublimation: Solid to gas Deposition: Gas to solid

A phase diagram usually displays changes in temperature on the horizontal axis and changes in pressure (in atmospheres, or atm) on the vertical axis. Lines drawn within the temperature-pressure field of the diagram represent the boundaries between phases, as shown for water and carbon dioxide in the figure.

Try an example: Ethanol (C2H6O) has a freezing point of –114 degrees Celsius. The compound 1-propanol (C3H8O) has a melting point of –88 degrees Celsius. At 25 degrees Celsius (where both compounds are liquids), which one is likely to have the higher vapor pressure, and why?

First, notice that a freezing point and a melting point are the same thing — that point is the temperature at which a substance undergoes the liquid-to-solid or solid-to-liquid phase transition. Next, compare the freezing/melting points of ethanol and propanol. Much colder temperatures must be achieved to freeze ethanol than to freeze propanol. This suggests that ethanol molecules have fewer attractive forces among themselves than propanol molecules do. At 25 degrees Celsius, both compounds are in liquid phase. In pure liquids whose particles have weaker intermolecular (between-molecule) attraction, the vapor pressure is higher because the molecules at the surface of the liquid can more easily escape into vapor (gas) phase. So, ethanol has the higher vapor pressure at 25 degrees Celsius.