t these substances in turn act upon each
other to form iron and steam once more. When these two opposite
reactions go on at such rates that the weight of the iron changed into
iron oxide is just balanced by the weight of the iron oxide changed into
iron, there will be no further change in the relative weights of the
four substances present in the tube. The reaction is then said to have
reached an equilibrium.

~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