So I'm trying to come up with some neat experiments to perform for some kids and I was thinking of showing them the effect of different temperature and pressure on the phase changes of water. Specifically, I was interested in subliming water by freezing it in a flask under reduced pressure and then having it go right into the gas phase after heating it up. The triple point diagram looks like this: http://www.chemguide.co.uk/physical/phaseeqia/pdh2o2.gif So I froze the water in dry ice down to about -70 C and then put the flask under vacuum. I let the flask warm up, but alas, it went back to the liquid phase rather than straight to the gas phase. What went wrong? Is it the vapor pressure of the water?