The pressures of gaseous mixtures

The rigid statement of the law of partial pressures, namely, that the pressure of a gaseous mixture is the sum of the contributions made by the several varieties of molecule actually present in it, does not readily lend itself to direct experimental proof. For practical purposes, Dalton’s law is usually taken to imply that the total pressure is the sum of those which can be experimentally measured by enclosing each constituent gas in turn by itself in the volume of the mixture. We shall refer to this as the law of additive pressures. It covers the case of gases which deviate from Boyle’s law, but it involves the assumption, duly stated by Dalton, that in a mixture each constituent completely retains its individual character, and does not by its presence affect the pressure of the others. Another frequently used form of Dalton’s original law is that the volume of a mixture is the sum of the volumes of the separate components, all measured at a fixed pressure. This can be called the law of additive volumes. It has often been implicitly extended to liquid mixtures, in which deviations from additive volume have been held to betoken chemical association.