Conservation of Energy and the Increase in Disorder. The total energy of the universe is constant. Energy can be transferred by collisions in chemical and nuclear reactions, by light waves and other radiation, and in many other ways. However, it can never be destroyed. As these transfers occur, the matter involved becomes steadily less ordered.
Heat consists of random motion and the vibrations of atoms, molecules, and ions.
The higher the temperature, the greater the atomic or molecular motion. Everything tends to become less organized and less orderly over time. Thus, in all energy transfers, the overall effect is that the energy is spread out uniformly. Examples are the transfer of energy from hotter to cooler objects by conduction, radiation, or convection and the warming of our surroundings when we burn fuels. Excerpted from National Research Council.