Critical point of carbon dioxide

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Stage 1 Stage 2 Stage 3

A supercritical fluid is a substance above its critical temperature and pressure. At the critical point the gas and the liquid phases become identical and the visible boundary between the two phases vanishes. In the supercritical environment only one phase exists. A supercritical fluid, as it is termed, is neither a true liquid nor a true gas and is best described as intermediate to the two extremes.

Carbon dioxide is a popular supercritical fluid and has a critical temperature of 31.1^° C and a critical pressure of 73.8 bar. A high pressure stainless steel view cell apparatus with a quartz window is used to demonstrate the critical point of carbon dioxide. (The view cell shown here has a glass tube inserted in the cell). The cell is filled with carbon dioxide and heated to the desired temperature.

In stage one carbon dioxide is in supercritical fluid state corresponding to a temperature above the critical point.

Stage two corresponds to the temperature very close to the critical point where the contents of the cell become cloudy caused by scattering of light due to the large density fluctuations. This phenomenon is known as critical opalescence.

Stage three represents a temperature slightly below the critical point where a faint meniscus appears separating the liquid and the gaseous phases which are in equilibrium with each other.

Critical point movie: the movie shows the gradual phase transition of carbon dioxide from a single phase supercritical state (stage 1) through the critical point (stage 2) to a two phase (gas-liquid) subcritical state (stage 3).