The existence of frigorific mixtures can be viewed as a consequence of the Gibbs phase rule, which describes the relationship at equilibrium between the number of components, the number of coexisting phases, and the number of degrees of freedom permitted by the conditions of heterogeneous equilibrium. Specifically, at constant atmospheric pressure, in a system containing C linearly independent chemical components, if C +1 phases are specified to be present in equilibrium, then the system is fully determined (there are no degrees of freedom). That is, the temperature and the compositions of all phases are determined. Thus, in for example the chemical system H 2 O-NaCl, which has two components, the simultaneous presence of the three phases liquid, ice, and hydrohalite can exist only at atmospheric pressure at the unique temperature of –21.2 °C . The approach to equilibrium of a frigorific mixture involves spontaneous temperature change driven by the conversion of latent heat into sensible heat as the phase proportions adjust to accommodate the decrease in thermodynamic potential associated with the approach to equilibrium.