Can matter undergo phase transitions?

...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:

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• 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.)