It may be somewhat surprising to learn that there is no fundamental distinction between a gas and a liquid. It was noted above that a gas occupies a volume about 1,600 times greater than that of an equal weight of liquid. The question arises as to the behaviour of a gas that has been compressed to 1/1,600 of its volume by application of sufficiently high pressure. If this compression is carried out above a specific temperature called the critical temperature, which is different for each gas, no phase change occurs, and the resulting substance is a gas that is just as dense as a liquid. If the compression is carried out at a fixed temperature below the critical temperature, an astonishing phenomenon occurs—at a particular pressure liquid suddenly forms. Attempts to compress the gas further simply increase the amount of liquid present and decrease the amount of gas, with the pressure remaining constant until all the gas has been converted to liquid. The applied pressure must subsequently rise a great deal to reduce the volume further, since liquids are much less compressible than gases.

The abrupt condensation of a gas to a liquid usually does not seem astonishing because it is so commonplace—nearly everyone has boiled water, for example, which is the reverse process. From the standpoint of the kinetic-molecular theory of gases, however, it is something of a mystery. Why does it occur so abruptly and only at temperatures below a critical temperature? Equations have been written down that describe condensation, but an explanation is still lacking in the sense that no one has been able to show that it must occur, given only the forces between the molecules and the fact that their motion is described by ordinary mechanics. Condensation, which is an example of a first-order phase transition, remains one of the outstanding unsolved problems of statistical physics.

The critical temperature marks the separation between an abrupt change and a continuous change. Other peculiar phenomena occur near the critical temperature. The densities of the coexisting liquid and gas (which is usually called a vapour in this case) become closer as the critical temperature is approached from below, and at the critical temperature they are identical. There is a unique point for every fluid, called the critical point. It is described by a critical temperature, a critical volume, and a critical pressure, at which liquid and vapour become identical. Above that temperature there is no distinction between gas and liquid; there is only a single fluid. Moreover, it is possible to pass continuously from an apparently definite gas or vapour to an apparently definite liquid with no abrupt condensation occurring. This can be accomplished by heating the vapour above the critical temperature while keeping the volume constant, then compressing it to a high density characteristic of a liquid, and finally cooling it at constant volume to its original temperature, where it is now clearly a liquid.

In short, gases and liquids are just the extreme stages of a fluid, with no fundamental distinction between the two. For this reason, an arbitrary decision has been made for the present discussion to define what is meant by the gaseous state. The definition will be based on the number density (i.e., molecules per unit volume): the number density of the fluid must be low enough that only collisions between two molecules at a time need to be considered. More specifically, the mean free path must be much larger than the molecular diameter. Such a fluid shall be termed a dilute gas.

A few brief historical remarks are in order before leaving the subject of the continuity of the gaseous and liquid states. The first extensive experimental study that clearly demonstrated the phenomena involved was performed on carbon dioxide, CO₂. (Carbon dioxide, whose solid form is called dry ice, has a critical temperature of 31° C.) The experiment was conducted by Thomas Andrews at what is now the Queen’s University of Belfast in Northern Ireland, and its results were summarized in 1869 in a Bakerian lecture to the Royal Society of London entitled “On the Continuity of the Gaseous and Liquid States of Matter.” In 1873 a Dutch thesis was presented to the University of Leiden by Johannes D. van der Waals with virtually the same title (but in Dutch) as Andrews’ lecture. In his study van der Waals used some ingenious approximations to obtain a simple equation relating the pressure, temperature, and molar volume of a fluid, based on a model that considered molecules as hard spheres with weak long-range attractive forces between them. This equation can be used to locate the critical point of a system, and it is also consistent with the occurrence of condensation when supplemented with a thermodynamic condition. This is possibly one of the most-quoted but little-read theses in science. Nevertheless, van der Waals started a scientific trend that continues to the present. His pressure-volume-temperature relation, called an equation of state, is the standard equation of state for real gases in physical chemistry, and at least one new equation of state is proposed every year in an attempt to improve on its quantitative accuracy (which is not very good). It furnished the impetus for the development of theories of liquids and of solutions. The equation is compatible with a unifying idea called the principle of corresponding states. This principle states that, if the pressure (p), volume (V), and temperature (T) of a gas are replaced, respectively, with the corresponding reduced variables—i.e., the pressure divided by the critical pressure (p/p_c), the volume divided by the critical volume (V/V_c), and the temperature divided by the critical temperature (T/T_c)—all gases will behave in essentially the same manner.

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The critical point has itself proved to be a rich and deep subject. The gas-liquid critical point turns out to be only one of many types of critical points, including those of a magnetic variety, with the common feature that long-range correlations develop regardless of the molecular details of the system. That is, any small part of a system near its critical point seems to “know” what quite distant parts are doing. The mathematical description of the behaviour of a system near its critical point also becomes rather unusual.