<P> The third law of thermodynamics is sometimes stated as follows, regarding the properties of systems in thermodynamic equilibrium: </P> <P> The entropy of a system approaches a constant value as its temperature approaches absolute zero . </P> <P> At absolute zero (zero kelvin) the system must be in a state with the minimum possible energy . Entropy is related to the number of accessible microstates, and there is typically one unique state (called the ground state) with minimum energy . In such a case, the entropy at absolute zero will be exactly zero . If the system does not have a well - defined order (if its order is glassy, for example), then there may remain some finite entropy as the system is brought to very low temperatures, either because the system becomes locked into a configuration with non-minimal energy or because the minimum energy state is non-unique . The constant value is called the residual entropy of the system . </P> <P> The Nernst--Simon statement of the third law of thermodynamics concerns thermodynamic processes at a fixed, low temperature: </P>

When is the entropy of a substance said to be zero
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