The most important physical law relevant to mass transport processes is the principle of conservation of mass. This principle will be applied to (i) the conservation of the total mass of a material, and (ii) the mass balance of a specific chemical species in a mixture. In this section, we will derive and apply total mass balance equations for arbitrary control volumes. The resulting equations are called macroscopic mass balances and they are useful to solve problems in physical domains that are well mixed; that is, where species concentrations are spatially uniform.