Phase fraction ($x_{\beta }=m_{\beta }/m$)

A phase fraction is the proportion of a mixture occupying a specified thermodynamic phase. State the basis: by mass, $x_{\beta }=m_{\beta }/{\sum }_{\alpha }{m}_{\alpha }$; by volume, divide the occupied volume by the total volume. These fractions need not agree when the mass densities differ. The latent heat heating curve uses the liquid fraction by mass, whereas liquid fraction in a mushy layer is defined by volume.