Phase Change to Supercritical CO2

In this short video, liquid CO2 is pumped into a windowed pressure vessel and while the pressure increases, the viewer watches the CO2 change phases past the CO2 critical point into the supercritical region. Supercritical CO2 is no longer distinguishable between a liquid or a gas. The CO2 pressure vessel is then depressurized, and the viewer can see the phases separate as the pressure is let down.

[lorenwilson]

Interesting video of moving from liquid to the supercritical region and back. To pass through the exact critical point, the volumes of the liquid and gas have to be equal as the temperature is slowly raised. Use the pump (heard in the background near the beginning of the video) to add or remove a bit of CO2 to keep the liquid-vapor interface in the middle. Rocking the vessel while slowly warming (1°C per minute or less) works really well to show the densities of the liquid and vapor phases approaching each other as the critical point is approached. You'll see lots of cloudiness, and sometimes a dark orange color, depending on the chemical.