[‹A›], [‹B›], [‹C›] and [‹D›] are used to represent the concentrations[166] of [p092] the four reacting substances and ‹k› is some definite number, called the equilibrium constant.
The law was discovered by Guldberg and Waage in 1867, and, with certain limiting conditions (see below) it has been fully established by extensive experimental work.[167] The significance of the law may be interpreted on the basis of the following considerations. If we start with the two substances ‹A› and ‹B› alone and have one mole of each in one liter (as gas or in solution) at a given temperature, then, all the conditions being given,—the temperature, the concentrations, and the nature of the substances,—the reaction ‹A› + ‹B› → ‹C› + ‹D›, leading to the formation of ‹C› and ‹D›, ‹will proceed with a perfectly definite velocity›. The molecules of ‹A› and of ‹B› move in all directions (kinetic theory of gases and solutions), and molecules of ‹A› will collide with molecules of ‹B› a definite number of times in unit time and will form a definite number[168] of molecules of ‹C› and ‹D› per minute. The velocity of chemical change of a given substance (‹chemical velocity›) is also measured in terms of moles, and is represented by the number of moles or the fraction of a mole changed per minute. If ‹v′›_{ 1} stands for the velocity of the action between ‹A› and ‹B›, under the given conditions, then
where ‹k›_{ 1} is some number. Now, if the concentration of one of the components, ‹e.g.› ‹A›, should be doubled, then the chances for collision and for action between molecules of ‹A› and ‹B› will be twice as great as before and the velocity of the action will be doubled. If only one-tenth of the concentration of ‹A› (one-tenth mole) is used, the velocity will only be one-tenth as great as originally, and, in general terms, if [‹A›] moles of ‹A› are used per liter, the ‹velocity of the change will be proportional to› [‹A›], ‹and equal to› ‹k›_{ 1} × [‹A›]. If the concentration of the other reacting component, ‹B›, is now doubled, the chances for action are again doubled, and, in general, the velocity of the action will be proportional also to the concentration [p093] [‹B›] of the second reacting substance. For the velocity, ‹v›_{ 1} of the action for any concentrations, [‹A›] and [‹B›], of ‹A› and ‹B› at any moment at a given temperature, we have
