23.3.1—Terms: entropy, S, as the number of possible

Entropy, S, describes the number of possible arrangements of particles and their energy. More accessible microstates correspond to greater entropy.

It is a system property, not simply “disorder”. Particle number, phase, temperature and mixing all affect the number of accessible arrangements.

A gas has much higher entropy than the same substance as a solid because particles can occupy many more positions and energy distributions.

Entropy is not a synonym for messiness and a reaction can increase one part of a system while decreasing another.