a separation or dispersal, the opposite of association. In chemistry the term is given to chemical reactions in which a substance decomposes into two or more substances, and particularly to cases in which associated molecules break down into simpler molecules. Thus the reactions NH 4 Cinh 3 +HC1, and PC PC13+C 12 are instances of the first type; N 2 O 4 2NO 2, of the second (see Chemical Action). Electrolytic or ionic dissociation is the separation of a substance in solution into ions (see Electrolysis; Solution).