Supercritical Carbon Dioxide
The lines on the diagram represent the temperatures and pressures at which two states can exist together in equilibrium. At the triple point, all three states co-exist.
As temperature and pressure are increased along the liquid/gas line, the distinction between these two states eventually disappears, and the phases become identical. This happens at the critical point. For carbon dioxide, it occurs at a pressure of 72.9 atmospheres but a temperature of only 31.1°C. Above this point the carbon dioxide is said to be in the supercritical state.
Supercritical fluids show properties of both liquids and gases. It will fill any size of container, like a gas, and dissolve materials like a liquid. Its power as a solvent can also be "tuned" very easily by changing the pressure.
For a movie clip of carbon dioxide being heated and compressed to its critical point, see the pages at the Leeds Cleaner Synthesis Group (University of Leeds, UK)
Carbon dioxide has the advantage over other supercritical fluids that its critical temperature is remarkably low at only 31.1°C, so high temperatures are not necessary. This means scCO2 can be used as a solvent with materials that would decompose at higher temperatures.
Making Carbon Dioxide