THE LAWS OF DEFINITE COMPOSITION AND DEFINITE PROPORTIONS H. G. DEMING University of Nebmska, Lincoln, N e b m s k a
DEFINITE composition arises from two distinct causes: (1) in molecular compounds, from union of a definite number of atoms of each element in forming a molecule; (2) in ionic compounds, from the fact that oppositely charged ions must be present in definite proportions in order that the substance as a whole may be electrically neutral. The law fails with many of the silicates because an indefinite number of silicon atoms in the silicate network (a "giant anion") may be replaced even by atoms of a diierent valence from silicon. The consequent alteration in the charge on the anion is then compensated by an alteration in the kind (or, sometimes, in the number) of the cations incorporated into the crystal structure. The law also fails whenever an indefinite number of the atoms of a crystal (1) may be withdrawn, without causing the structure to collapse, (2) may be replaced by other atoms, or (3) may accommodate foreign atoms between them. Thus, three different types of solid solutions are produced, which have only a definite limiting composition, perhaps expressible by a definite formula. When reactants and resultants are all of definite composition, i t is commonly assumed that a given reaction, unaccom@nied by others, will transform them in definite proportions by weight. The words in italics require explanation, since even so simple a reaction as the direct union of hydrogen and oxygen to form water involves several successive steps, and may proceed by several diierent mechanisms a t once. Let s diierent substances be concerned in a reaction.
Then s - 1 definite ratios must be established between the weights (or numbers of molecules) of the reactants, if the law of definite proportions is to hold. We also have x independent equations, each stating that the total weight (or total number of atoms) of some element or group of elements remains constant. For a unique solution, the number of variables (weight ratios) to be determined must equal the number of independent equations, hence s = z 1. In other words, the law holds, and the reaction may be represented by a balanced equation, whenever the number of substances appearing in the equation exceeds by one the number of independent elements or undecomposed radicals. Independent elements are those not brought into balance in the process of balancing with respect to other elements. In the decomposition of calcium carbonate into calcium oxide and carbon dioxide, there are only two independent elements, since oxygen is found to be in balance when the equation has been balanced with respect to calcium and carbon. In displacement or exchange reactions, only three of the four elements or radicals displaced or exchanged are independent. By this test the law of definite proportions is found to hold for all of the simplest types of chemical reactions (direct union, decomposition, internal rearrangement, displacement, exchange) and for their combination in consecutive reactions, whenever the intermediate steps are completed. It fails when the simple reaction types occur simultaneously or when intermediate steps remain uncompleted.
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