Some flows involve a mixture of two physical phases.
Such flows are called two-phase flows. The concept of the partial density of each phase of a two-phase mixture is a useful one for analyzing such flows. The partial density of a phase is simply the ratio of the mass of the phase in a sample of fluid to the volume of that sample. In this sense, the phase density is directly analogous to the partial density of a chemical specie in a mixture. Sometimes the amount of the phase can be determined as the fraction of the volume of the fluid that is occupied by the phase.