# CO2 sublimation in a closed container

1. Mar 25, 2016

### Jonhorde

A coworker posed a thought experiment; if you sealed a sample of frozen CO2 in an uncompressable container of identical volume and allowed it to warm to room temperature, how could it melt/evaporate, since there would be no room for the gas to expand
My guess is that it would sublimate to a very compressed gas, since, running the thought backwards, I know you can't compress a gas into a solid, (short of Neutron star pressures).
I see on the CO2 phase diagram a supercritical fluid at very high pressures, how could I estimate the pressure it would be under?

2. Mar 25, 2016

### TeethWhitener

This is (kind of) correct, depending on the temperature and volume of the container. If the temperature is large enough and the volume is small enough, it might become a supercritical fluid--a fluid that doesn't have sharp phase changes like boiling/condensation in standard liquids or gases--instead.
This is not correct. It is absolutely possible to compress a gas to a solid. For example, in the case of CO2, the triple point is at 5.1 atm and -56.7°C. This means that if you cool CO2 gas to -56.7°C and then apply a pressure of 6 atmospheres, the gas will become a solid.
You'll have a supercritical fluid any time you're above the critical temperature and pressure:
https://en.wikipedia.org/wiki/Supercritical_carbon_dioxide

3. Mar 25, 2016

### jbriggs444

To estimate the pressure, you have to match the right density. A quick trip to Google says that the density of the solid state is in the neighborhood of 1.4 to 1.6 grams per cc. Another quick trip to Google finds an online calculator that says that you can get a density of 1.4 g/cc if you push the pressure up to 5000 bars and 1.5 g/cc at 8000 bars, both at room temperature.

http://www.energy.psu.edu/tools/CO2-EOS/index.php

4. Mar 26, 2016

### Jonhorde

Hmm, thank you.
Just to make sure I understand,
A sample of solid CO2 with no room to expand in an uncompressable container, wouldn't melt left at room temperature, but would remain solid with an internal pressure approaching 8000bar?