The chemical potential is a partial molar Gibbs free energy , defined as µi = ( G/ ni)T,P,nj. The definition means that the chemical potential is the change in Gibbs free energy when one mole of a substance is added to a very large amount of a sample. Chemical potential is a measure of chemical stability that can be used to predict and interpret phase changes and chemical reactions. Substances with higher chemical potential will react or move from one phase to another to lower the overall Gibbs free energy of the system. For example, consider a mixture of ice and water. If the ice melts, the chemical potential of the water was lower than that of the ice. If the water freezes, the chemical potential of the ice was lower.