Question

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

Answer 1

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

Consider the phase diagram for `"CO"_2`:

This says that a sublimation phase change could happen, for example, at a constant `"1 atm"` by increasing the temperature past about `-78.5^@ "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^@ "C"`, and decrease the pressure past `"1 atm"` to sublime as well.

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