The second law says the entropy of the system plus its surroundings has to rise. Pick a change, slide the temperature, and watch which ledger pays for the other. The third law curve below shows where the whole scale starts.
At 0 K a perfect crystal has one microstate, so S = k ln 1 = 0. That true zero is why a table can print an absolute S° for every substance.
| ΔH of the system | |
| ΔS of the system | |
| −ΔH / T, the surroundings | |
| sum of the two | |
| turning point |