conservation of energy

Principle that states that in a chemical reaction, the total amount of energy in the system remains unchanged. Energy can be transferred from one form into another but cannot be created or destroyed.

In a chemical reaction, for each component there may be changes in energy due to change of physical state, changes in the nature of chemical bonds, and either an input or output of energy. However, there is no net gain or loss of energy. This is true outside of chemical reactions; for example the chemical energy of a battery is transferred into the electrical energy in a circuit, which can then be transferred into the heat and light energy of a bulb, but the total amount of energy is constant.