(Chemistry term) Occuring when the pressure and temperature of a substance exceed its critical pressure and critical temperature; disparate phases of the substance will merge into one and become indistingushable. Supercritical fluids fill their containers like gases but share properties with both gases and liquids.

You can make a graph of the boiling point of a liquid as a function of pressure. This type of graph is known as a phase diagram.

Above the curve, the substance is a gas. Below the curve, the substance is a liquid. At any point on the curve, gas and liquid are in equilibrium. besides indicating the boiling point of a liquid at a given pressure, this graph also gives you the vapor pressure at any given temperature.

Well, maybe not any temperature. As you follow this curve up and to the right, it ends in a critical point, which is not to be confused with the triple point. The pressure of the critical point is the critical pressure, and the temperature at the critical point is (ready for a big surprise?) the critical temperature. Beyond the critical point, the substance is called a supercritical fluid, since there is no longer any distinction between liquid and gas.

The critical point is an interesting location on the phase diagram...the liquid and vapor become identical, so you have many interesting thermodynamic properties....infinite heat capacity, zero surface tension, etc. Mind you, there are a few practical applications for supercritical fluids.

Suppose you're feeling particularly evil. You take a bunch of coffee beans and put them in a pressurized chamber full of liquid carbon dioxide. You then heat it up to the critical temperature while maintaining enough pressure to prevent the carbon dioxide from boiling. Let it soak for a while, then release the carbon dioxide through an escape valve. If all goes according to plan, the expanding gas will give you a lovely little snowstorm...except the "snowflakes" are almost pure caffeine. This happens because the caffeine is somewhat soluble in carbon dioxide near the critical point, but less soluble at lower pressures. The oils responsible for the aroma of coffee aren't nearly as soluble. This means the beans are now decaffeinated but haven't lost their flavor (like decaffeination with water causes) and aren't contaminated with methylene chloride (Which dissolves caffeine, leaves the flavor behind, and maybe causes liver problems. But you don't want to be that kind of evil...).

So, you now have your own stash of caffeine, and some decaffeinated coffee beans which will leave your co-workers wondering how they built up such a tolerance so suddenly.

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