Julian, that's the complex answer. Remember PV=nRT?
This is what I use...
Change it to PV/T=nR
From this you can write:

(P1V1)/T1=(P2V2)/T2

Julian Scarfe wrote:

It does. For a phase change, there's an equation called the Clapeyron
equation that shows how the temperature varies with pressure.

dT/dP = T deltaV / deltaH

where deltaV is the volume change and deltaH is the enthalpy change (think
of it as energy and you're not too far off) of the phase change.

(It sort of makes sense that the pressure variation of the phase change
depends on what the volume change is. If there's no volume change it
doesn't really matter what the pressure is, as everything happens in the
same space.)

For water - steam, deltaV is very large, so the variation of T with P is
significant.
For ice-water, deltaV is tiny, so the variation of T with P is miniscule.

At about 130 atmospheres, the melting temperature of water drops by 1 degC.

Julian Scarfe