A fluid mixture is defined as any fluid which contains a solute which alters its thermal properties from those of the pure uncontaminated fluid. Sea water and moist air are examples.
In general, the unique specification of the thermal state of a fluid mixture (unlike a pure fluid) requires the enumeration of at least three independent thermal properties. For example, the thermal state of a small parcel of sea water can be specified by its temperature, pressure, and the concentration (weight per unit volume) of each of its major constituents. However, it is found that this is more stringent than is necessary in the case of oceanic waters, since the relative concentrations of the major constituents appears to be essentially invariant. Consequently, for all practical purposes, the number of required properties for specifying the thermal state of sea water is three. Temperature, pressure and salinity are possible choices; however, any three independent thermal properties will equally well specify its thermal state. Thus bulk viscosity, sound velocity, and refractive index specify the thermal state of sea water uniquely since all other thermal properties can be ascertained from the three which are specified.