critical point

1. Physics. the point at which a substance in one phase, as the liquid, has the same density, pressure, and temperature as in another phase, as the gaseous.

2. Math.

a. (of a function of a single variable) a point at which the derivative of the function is zero.

b. (of a function of several variables) a point at which all partial derivatives of the function are zero.

[1875-80]

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In science, the set of conditions under which a liquid and its vapour become identical.

The conditions are the critical temperature, the critical pressure, and the critical density. If a closed vessel is filled with a pure substance, partly liquid and partly vapour, and the average density equals the critical density, the critical conditions can be achieved. As the temperature is raised, the vapour pressure increases, and the gas phase becomes denser while the liquid expands and becomes less dense. At the critical point, the densities of liquid and vapour become equal, eliminating the boundary between the two.

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▪ phase change

      in physics, the set of conditions under which a liquid and its vapour become identical (see phase diagram). For each substance, the conditions defining the critical point are the critical temperature, the critical pressure, and the critical density.

      This is best understood by observing a simple experiment. If a closed vessel is filled with a pure substance, partly liquid and partly vapour, so that the average density equals the critical density, the critical conditions can be achieved. As the temperature is raised, the vapour pressure increases, and the gas phase becomes denser. The liquid expands and becomes less dense until, at the critical point, the densities of liquid and vapour become equal, eliminating the boundary between the two phases. If the average density at the start is too low, all the liquid will evaporate before the critical temperature is reached. If the initial average density is too high, the liquid will expand to fill the container.

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