What is entropy? What is its significance for engineering devices and processes? Solution Entropy is a thermodynamic quantity representing the unavailability of a system\'s thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system. The entropy of a substance is real physical quantity and is a definite function of the state of the body like pressure, temperature, volume of internal energy. Significance- the significance of entropy revolves around the stability of a system/reaction. the higher the entropy, the more unstable the system/reaction. from the third law of thermodynamics we know that the entropy of a crystalline solid is zero at the absolute temperature (0 K). thus this is the most stable form of a substance. An increase in entropy implies a transition from on ordered to a less ordered state of affair. .