~Factors which determine the point of equilibrium.~ There are two factors which have a great deal of influence in determining the point at which a given reaction will reach equilibrium. 1. _Influence of the chemical nature of the substances._ If two reversible reactions of the same general kind are selected, it has been found that the point of equilibrium is different in the two cases. For example, in the reactions represented by the equations 3Fe + 4H_{2}O <--> Fe_{3}O_{4} + 8H, Zn + H_{2}O <--> ZnO + 2H, the equilibrium will be reached when very different quantities of the iron and zinc have been changed into oxides. The individual chemical properties of the iron and zinc have therefore marked influence upon the point at which equilibrium will be reached. 2. _Influence of relative mass._ If the tube in which the reaction 3Fe + 4H_{2}O <--> Fe_{3}O_{4} + 8H has come to an equilibrium is opened and more steam is admitted, an additional quantity of the iron will be changed into iron oxide. If more hydrogen is admitted, some of the oxide will be reduced to metal. The point of equilibrium is therefore dependent upon the relative masses of the substances taking part in the reaction. When one of the substances is a solid, however, its mass has little influence, since it is only the extent of its surface which can affect the reaction. ~Conditions under which reversible reactions are complete.~ If, when the equilibrium between iron and steam has been reached, the tube is opened and a current of steam is passed in, the hydrogen is swept away as fast as it is formed. The opposing reaction of hydrogen upon iron oxide must therefore cease, and the action of steam on the iron will go on until all of the iron has been transformed into iron oxide. On the other hand, if a current of hydrogen is admitted into the tube, the steam will be swept away by the hydrogen, and all of the iron oxide will be reduced to iron. _A reversible reaction can therefore be completed in either direction when one of the products of the reaction is removed as fast as it is formed._ ~Equilibrium in solution.~ When reactions take place in solution in water the same general principles hold good. The matter is not so simple, however, as in the case just described, owing to the fact that many of the reactions in solution are due to the presence of ions. The substances most commonly employed in solution are acids, bases, or salts, and all of these undergo dissociation. Any equilibrium which may be reached in solutions of these substances must take place between the various ions formed, on the one hand, and the undissociated molecules, on the other. Thus, when nitric acid is dissolved in water, equilibrium is reached in accordance with the equation H^{+} + NO_{3}^{-} <--> HNO_{3}.