Chemical reaction
![A thermite reaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake.](/Images/godic/202501/04/ThermiteReaction2529.jpg")
![Antoine Lavoisier developed the theory of combustion as a chemical reaction with oxygen](/Images/godic/202501/04/Antoine_lavoisier_color2529.jpg")
![As seen from the equation CH4 + 2 O2 → CO2 + 2 H2O, a coefficient of 2 must be placed before the oxygen gas on the reactants side and before the water on the products side in order for, as per the law of conservation of mass, the quantity of each element does not change during the reaction](/Images/godic/202501/04/Combustion_reaction_of_methane2529.jpg")
![Isomerization of azobenzene, induced by light (hν) or heat (Δ)](/Images/godic/202501/04/Azobenzene_isomerization_de.svg2529.png")
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes may occur.