# During a phase change, what happens to the temperature of a substance?

Jun 7, 2017

We don't really know, because there are two common types of phase changes...

Consider the phase diagram for ${\text{CO}}_{2}$:

This says that a sublimation phase change could happen, for example, at a constant $\text{1 atm}$ by increasing the temperature past about $- {78.5}^{\circ} \text{C}$.

That is a horizontal phase transition, with a change in temperature at constant pressure.

But we could also keep the temperature constant at $- {78.5}^{\circ} \text{C}$, and decrease the pressure past $\text{1 atm}$ to sublime as well.

That is a vertical phase transition, with a change in pressure at constant temperature.