A reaction is favored if entropy increases: There is also a bias in nature toward increasing entropy in a system. Reactions can happen when entropy increases. If there is no difference in the final and initial state of entropy then entropy will be zero. Devices with a steady state of flow of energy like nozzles, and turbines have zero entropy. Reversible processes also have zero entropy. In fact, if we look at the Universe today, even though the overall entropy is enormous, the fact that the volume is so large drives the entropy density to a relatively small number: about ~1027 kB/m3to 1028 kB/m3. The heat that Earth radiates into space has a much higher entropy than the incoming visible light energy we receive from the sun. The low-entropy light coming in is useful energy while the high-entropy heat leaving is essentially useless.