Thermochemistry is a subject in which the changes in internal heat of the reacting products is studied. Every explosion is a series of several reactions that take place in a particular sequence at extremely rapid speed. These reactions are invariably exothermic, i.e. heat is evolved in the reaction. In an explosion the ingredients of the chemicals, or mixtures of them, first decompose into elemental form and again recombine by consuming some of the energy evolved. Energy changes in explosive reactions are calculated either from known chemical laws or by analysis of the products.