The first law of thermodynamics is the law of conservation of energy. During a change of the system (from an initial state to final state), the total energy of system and surrounding remains constant. Thus, energy is conserved during change.
Following are the statements of the first law of thermodynamics.
1) The total internal energy of an isolated system is constant. 2) Energy can be converted from one form to another. But it cannot be created or destroyed.
3) The total energy of the universe is constant.
4) When a particular amount of energy disappears, an equivalent amount of energy (in other forms) appears.
5) The total energy of the universe must remain constant even-though it can change from one form to another.