GITAM, Department of Engineering Physics


Entropy and Disorder

            With an increase in entropy, the thermal agitation and hence disorder of the molecules or substance increases, ie., growth of entropy implies a transition from order to disorder. Thus the principle of increase of entropy is intimately connected with the less ordered state of affairs. According to it, a high entropy system should be in great disorder.

            When ice is converted into water and then into steam, the entropy of disorder of molecules increases. On the other hand, when the steam is converted into water and then into ice, the entropy and disorder of molecules continuously decreases. Thus when the temperature of the system is lowered the amount of entropy and disorder in it decreases. Entropy of a substance is, therefore, said to be a measure of degree of disorder prevailing among its molecules, just as temperature is the measure of degree of hotness of a substance.

            At the absolute zero temperature, the thermal motion completely disappears so that the disorder and hence entropy tends to zero and the molecules of a substance are in perfect order, ie., well arranged ( Third law of thermodynamics).