You gained two pounds while on vacation. which of the following has changed?
a only your mass has changed
b only your weight has changed
c neither your mass nor your weight has changed
d both your mass and your weight has changed

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.