Consider the southeast coast of North America and the northwest coast of Africa. According to the current theory of plate tectonics, these two regions were once very close together. However, the coasts do not quite fit together like puzzle pieces, as one might expect. This can be explained by the fact that the southeast edge of North America's actually does fit nicely with the northwest edge of Africa's even if the coasts do not.

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition 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.