It underpins several techniques to evaluate reaction constants and affinities. The third law has been useful to physicists in understanding heat capacities and other response functions at low temperature, and it proved to be very useful and influential in the foundations of chemistry. It is simply that the third law is a useful summary: like any law of physics, it is at once a generalization from empirical observation, and also part of a self-consistent and elegant theoretical framework. However, this does not mean that we cannot determine absolute entropy without the third law. The fact that conditions of zero entropy are not self-evident is also the reason why the third law of thermodynamics is introduced. For this reason, the term 'absolute entropy' is introduced to draw attention to those cases where we are interested in the entropy itself, not just the change. However, whereas it is relatively straightforward to measure total mass or total volume, and to understand the case of zero mass or volume, it is less straightforward to measure total entropy and to understand the case of zero entropy. A similar statement applies to other extensive properties such as mass and volume. In many situations, one does not need to know the total entropy, only the entropy change in some given process. Absolute entropy is simply the total entropy of a given system, usually indicated by the symbol S. The aim of this paper is to clarify some issues in classical thermodynamics, concerning absolute entropy and the third law of Thermodynamics.

Why are entropies absolute