<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2010) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2010) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 . </P> <P> When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an energy change as new products are generated . An example of a chemical change is the reaction between sodium and water to produce sodium hydroxide and hydrogen . So much energy is released that the hydrogen gas released spontaneously burns in the air . This is an example of a chemical change because the end products are chemically different from the substances before the chemical reaction . </P>

Causes materials to form new materials through a chemical change