Nitrogen dioxide is a gas that can be generated by emissions from vehicles and factories. It can also be generated by natural sources, such as forest fires, lightning, and microbial activity in soil. The equation for producing nitrogen dioxide is shown below.
2NO+ 02
+ 2NO
Colorless
Colorless
gas
Brown
gas
gas
Which of these provides evidence that a chemical reaction occurs?
O A. A gas is present
OB. The total number of atoms remains unchanged.
a C. The state of matter remains the same.
D. A brown gas is produced.

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.