A gas species, , is absorbed by a solvent, , containing a solute, . The gas-liquid interface is at . Assume that the liquid phase concentration of at is equal to 1 gmol/liter. The concentration of species in the solvent at is chosen to be equal to 4 gmol/liter. An instantaneous irreversible chemical reaction takes place between and (). The species , , and are present in low concentrations and Fick's second law applies. The diffusivities of species and in are taken to be and , respectively. Because the chemical reaction between and is considered as instantaneous, there is an interface parallel to the plane where neither nor is present. The position of this interface increases with time as is used up by the chemical reaction. This Demonstration displays the position of this interface and the concentration of species and (shown in red and blue, respectively). The computations of the interface position and the species concentrations are based on an analytical solution derived by Bird et al. (see reference below for details).