During photosynthesis, plants incorporate atmospheric Cinto more complex energy-storage
molecules that increase the mass of the plants. Plants need sunlight for this process because plants transform light
energy into
energy stored in the molecules.

carbon dioxide

oxygen

**

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.