A chemical change involves the formation of new substances with different properties. Souring of milk occurs due to the conversion of lactose into lactic acid by bacteria, altering its chemical composition. Crystallization of sodium chloride is a physical process where a solid separates from a solution in a regular crystalline structure, but the chemical identity of NaCl remains unchanged. Melting of ice is a phase transition from solid to liquid water, a physical change that does not alter the chemical formula H2O.