Question

Can matter undergo phase transitions?

Answer 1

...If it is matter, then I hope so. If it is matter, it should be describable by a phase diagram (at least, in principle).

Benzene is a fine example:

• If we start at `"280 K"` and `"30 torr"`, and increase the pressure at constant temperature past roughly `"40 torr"`...

...We condense the gas into a liquid. (If we reverse the process we just did, we vaporize the liquid into a gas.)

• If we start at `"273 K"` and `"10 torr"`, and increase the pressure at constant temperature, we move vertically upwards in the phase diagram.

We consequently transform the gas into a solid, i.e. deposition, past roughly `"30 torr"`. (If we reverse the process, we perform sublimation.)

• And if we look closely, the solid-liquid coexistence curve has a negative slope (is not perfectly vertical).

So if we start at roughly `"280 K"` and `"40 torr"` (to the left of the triple point) and increase the pressure at constant temperature, past roughly `"50 torr"` we would melt the solid into a liquid. (The reverse process would be freezing the liquid.)