Entropy
A measure of the dispersal of energy and the number of microscopic arrangements available to a system.
Formula / equation
S = k ln W
Unit
J/(mol·K)
In depth
Boltzmann's equation links the macroscopic property to the number of microstates W. Entropy rises with temperature, with volume for a gas, with the number of particles, and on melting or boiling. Reactions that increase the number of gas molecules always show a substantial entropy gain.
Examples in the real world
Water's entropy rises from 41 J/(mol·K) as ice to 70 as liquid to 189 as vapour.