When Maya visited the Grand Canyon on a school trip she made some observations while standing at the top of the canyon. She saw the river rushing by at the bottom and trees growing in cracks along the cliffs. She felt how windy it was at the top and how cold it got at night. She heard rocks fall and roll down the cliffs to the bottom. Which of Maya’s observations is an example of only erosion taking place?

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.