Mathematical modelling of chemical reaction networks usually focuses on what happens to the concentrations of the various chemicals involved as time passes. Following the example above, let a represent the concentration of H 2 {\displaystyle {\ce {H2}}} in the surrounding air, b represent the concentration of O 2 {\displaystyle {\ce {O2}}} , c represent the concentration of H 2 O {\displaystyle {\ce {H2O}}} , and so on. Since all of these concentrations will not in general remain constant, they can be written as a function of time e.g. a ( t ) , b ( t ) {\displaystyle a(t),b(t)} , etc.