How is the absolute entropy of a system determined according to the third law of thermodynamics?

The absolute entropy of a system is determined by referencing absolute zero, which is central to the third law of thermodynamics. According to this law, as a system approaches absolute zero (0 Kelvin), the entropy of a perfect crystalline substance approaches zero. This establishes a baseline from which absolute entropy can be measured for other states, meaning that the absolute entropy of any system can be calculated by integrating the heat capacity over temperature changes from absolute zero to the temperature of interest.

This concept is critical because it allows for a relative comparison of entropy values between different substances and states. The foundation laid by the third law helps scientists and engineers quantify the disorder or randomness in a system, facilitating calculations in thermodynamics and physical chemistry.

While other factors, such as temperature and molecular structure, play roles in defining properties of substances or in calculating changes in entropy, they do not serve as the primary reference point for absolute entropy as defined by the third law. Therefore, referencing absolute zero is essential for understanding how absolute entropy is established.