Dissolving of a solid substance in a liquid solvent as a function of time is proportional to two factors: (1) the difference between the concentration and the saturation concentration of the substance and (2) the quantity of undissolved solid in the solvent. Here the saturation concentration of the substance is taken to be 1 part of solute in parts of solvent. The variables are the mass of solute, the mass of solvent, the time to saturation , and the observed concentration .