Mass action in science is the idea that a large number of small units (especially atoms or molecules) acting randomly by themselves can in fact have a larger pattern. For example, consider a cloud of gas is moving in a given direction. Individual molecules will move in a semi-random walk, but if taken has a whole, they have direction.

The Law of Mass Action states that the rate of a chemical reaction is proportional to the concentrations of the reacting substances and the demand for their products. If the reactants are plentiful and the products in high demand, the reaction will occur quickly. If they are not, then the reaction will occur slowly.