Chemical changes occur when a substance combines with another to form a new substance, called
synthesis or, alternatively,
decomposes into two or more different substances. These processes are called
chemical reactions and, in general, are not reversible except by further chemical reactions. Some reactions produce
heat and are called
exothermic reactions and others may require heat to enable the reaction to occur, which are called
endothermic reactions. Understanding chemical changes is a major part of the science of
chemistry.