The third law of thermodynamics states that the entropy of a system at absolute zero is a well-defined constant.

The 3rd law was developed by the chemist Walther Nernst during the years 1906–12, and is therefore often referred to as Nernst's theorem or Nernst's postulate. If the composite of two thermodynamic systems constitutes an isolated system, then any energy exchange in any form between those two systems is bounded. There also exists a formulation of the Third Law which approaches the subject by postulating a specific energy behavior: It was proven in 2017 by Masanes and Oppenheim. It is impossible for any process, no matter how idealized, to reduce the entropy of a system to its absolute-zero value in a finite number of operations. Here a condensed system refers to liquids and solids.Ī classical formulation by Nernst (actually a consequence of the Third Law) is: The entropy change associated with any condensed system undergoing a reversible isothermal process approaches zero as the temperature at which it is performed approaches 0 K. The Nernst–Simon statement of the third law of thermodynamics concerns thermodynamic processes at a fixed, low temperature: The constant value is called the residual entropy of the system. Entropy is related to the number of accessible microstates, and for a system consisting of many particles, quantum mechanics indicates that there is only one unique state (called the ground state) with minimum energy. If the system does not have a well-defined order (if its order is glassy, for example), then in practice there will remain some finite entropy as the system is brought to very low temperatures as the system becomes locked into a configuration with non-minimal energy. The entropy of a perfect crystal at absolute zero is exactly equal to zero.Īt absolute zero (zero kelvin), the system must be in a state with the minimum possible energy, and the above statement of the third law holds true provided that the perfect crystal has only one minimum energy state. The list of property identifiers needed in the calling arguments and instructions are available in the Thermophysical Function help.The third law of thermodynamics is sometimes stated as follows, regarding the properties of systems in equilibrium at absolute zero temperature: The JANAF table reference for entropy is based on the Third Law of Thermodynamics which references the entropy of all pure crystalline substances to zero at absolute zero temperature. However, all ideal gas substances (which have a chemical symbol name, e.g., N2, CO2, CH4) have enthalpy values corresponding to JANAF table references. The reference state upon which the value of enthalpy is based varies with the substance.

Temperature must be the only argument, in addition to the substance name. The specific entropy of incompressible substances is a function of only temperature. Note also that for substance AirH2O (psychrometrics), the specific entropy returned by this function is the entropy of the air and water vapor mixture per unit mass of dry air. The remaining two can be any of the following: temperature (T), enthalpy (H), internal energy (U), relative humidity (R), humidity ratio (W), wetbulb (B), or dewpoint (D). One of these arguments must be total pressure (P). For all pure substances, the entropy function always requires two arguments, in addition to the substance name.įor AirH2O, three arguments are required. The value and units of the returned value depends on the Unit System setting. ENTROPY returns the specific entropy of a specified substance.

Absolute third law entropy of mixture