In a change the identity of the substance is
no changed, but only how the molecules of the substance are arranged
when the identity of a substance changes from compound to another a change takes place.

physical - arrangement of molecules changes

chemical - identity changes

Explanation:

the answer is c.the valence of the atoms combine .

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.