Which law states that "Energy cannot be created or destroyed, but may be changed from one form to another"?

"The law of conservation of energy is a physical law that states energy cannot be created or destroyed but may be changed from one form to another. Another way of stating this law of chemistry is to say the total energy of an isolated system remains constant or is conserved within a given frame of reference.

In classical mechanics, conservation of mass and conversation of energy are considered to be two separate laws. However, in special relativity, matter may be converted into energy and vice versa, according to the famous equation E = mc². Thus, it's more appropriate to say mass-energy is conserved."

-Anne Marie Helmenstine, Ph.D.