John is outstanding forward on his club select soccer team. He is know for his ball control and speed. After a game he often makes a sketch of moves that were successful. The dotted line on his sketch shows the direction the ball was moving before he kicked it.

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.