Reversible reactions. The reactions so far considered have been represented as continuing, when once started, until one or the other substance taking part in the reaction has been used up. In some reactions this is not the case. For example, we have seen that when steam is passed over hot iron the reaction is represented by the equation
3Fe + 4H2O = Fe3O4 + 8H.
On the other hand, when hydrogen is passed over hot iron oxide the reverse reaction takes place:
Fe3O4 +8H = 3Fe + 4H2O.
The reaction can therefore go in either direction, depending upon the conditions of the experiment. Such a reaction is called a reversible reaction. It is represented by an equation with double arrows in place of the equality sign, thus:
