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How to Make a Supercritical FluidWater Heated in Open Container

Water Heated in an Open Container

The temperature at which the vapor pressure above a liquid is one atmosphere is known as the boiling point. For water, the boiling point is 100°C or 212°F. If liquid water is placed in an open container, its temperature cannot be raised above 100°C, because this would cause the pressure of the water vapor to rise above one atmosphere, exceeding the surrounding atmospheric pressure.

Water Heated in a Sealed Container
If water is placed in a sealed container, however, then it can be heated to temperatures above 100°C, since there is no limit on the pressure of the water vapor.Water Heated in Closed Container

Densities Equalize at Critical Temperature & Pressure
As heat is applied to the sealed container, the density of the liquid water decreases through thermal expansion. Simultaneously, the density of the water vapor increases. We can continue this heating process, reducing the liquid's density and increasing the vapor's density until the two densities become equal.

The temperature at which the liquid and the gas densities become equal is called the critical temperature. Since the temperature and density inside the closed container are equal throughout it, the laws of thermodynamics require that the pressure inside the container also be equal throughout. This pressure is called the critical pressure.

Supercritical FluidSupercritical Fluid
A liquid that has been brought to conditions above its critical temperature and pressure is known as a supercritical fluid. For pure CO2, the critical conditions of pressure and temperature are 1074 psi and 31.1°C.

May 21/2008

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