Match each front with its description. occluded front
when a warm air mass slides over a cold air
mass
stationary front
when a cold air mass slides under a warm
air mass
cold front
when two cold fronts force a warm front up
and block it from the surface
warm front
when a cold air mass and a warm air mass
meet and do not move

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.