Critical State

CRITICAL STATE, a special state characterized by the disappearance of the difference between a liquid and its saturated vapor. The temperature, volume, and pressure corresponding to this state are also called critical. If a gas is compressed at a constant temperature, its elasticity (pressure) will increase. At a certain pressure and corresponding volume, a saturated vapor is obtained. With further compression, the pressure ceases to increase, but part of the vapor condenses into liquid. This condensation, during which the pressure remains constant, will obviously continue until all the vapor turns into liquid. The critical temperature was discovered by Mendeleev while studying the liquid state (see Liquids) and was named by him the temperature of absolute boiling. According to Mendeleev, this is the temperature at which a liquid loses its cohesive force. Extremely clear representation is given by the compression lines of gas and liquid at a certain temperature, so-called isotherms (see Liquids). In these phenomena, it is seen that the regions corresponding to the existence of liquid become smaller and smaller with increasing temperature, and finally at a certain temperature we have such a state that the difference between the liquid and the vapor it forms disappears, and the transition of liquid into vapor is continuous. This moment, observed on the diagram, is shown in the article Liquids (see) and corresponds to the critical temperature. The critical temperature plays an enormous role in the production of liquefied gases (the critical temperature is often defined as the temperature above which a gas cannot be converted into a liquid by any pressure).