Video clip 5 - The phase changes of SF6
This video footage of the inorganic molecule SF6 demonstrates the principles of a supercritical fluid.
Initially the chemical is in a liquid state filling the lower half of a fixed volume autoclave. As the temperature and pressure to the cell is increased/adjusted the SF6 begins to boil and evaporate into the upper half of the cell – the gaseous phase.
As we reach the critical temperature and pressure of SF6 (16.7 oC and 58.4 bar) the upper gaseous state incresingly becomes more dense and the lower liquid states becomes less dense. During this time a blurring of the meniscus occurs. Eventually a phase with properties intermediate between that of a gas and liquid is achieved, i.e. the supercritical phase.
As the video clip continues we lower the temperature and pressure back to the original values, once below 16.7 oC and 58.4 bar, the SF6 is no longer supercritical, and the gaseous phase crashes out. The meniscus returns and the autoclave reverts to a two phase system.
The critical temperature and pressure of carbon dioxide is higher than that of SF6, 31.1 oC and 73.8 bar.
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