Third Law of Thermodynamics
The entropy of a perfect crystalline substance approaches zero as the temperature approaches absolute zero.
Formula / equation
S → 0 as T → 0 K
Unit
J/(mol·K)
In depth
At absolute zero a perfect crystal has only one possible microscopic arrangement, so its statistical entropy is zero. This provides an absolute reference point, which is why entropies — unlike enthalpies — can be tabulated as absolute values rather than as changes.
Examples in the real world
Standard molar entropies such as 70 J/(mol·K) for liquid water are absolute values measured from this zero.