# How can i explain how water’s entropy can decrease when it freezes without violating the second law of thermodynamics? specifically, what happens to the entropy of its surroundings?

Apr 25, 2015

Ok, this is complicated....
I would say that when water freezes it get to a more ordered configuration (solid in a crystalline form); so consider that, among others, Entropy $S$ is a measure of disorder! Entropy increases with disorder.
The problem is the mathematical part to describe this decrease...
You need (in order to evaluate Entropy) of a Reversible process. I would use an isothermal process that takes reversibly (Veeeeeeeery slowly and infinitesimally) your water through the freezing process.

Take a certain amount of water and consider the heat $Q$ needed to freeze it (through the use of Latent Heat for example). Temperature $T$ during the process is constant (isothermal).

The change in Entropy will be: $\Delta S = \frac{Q}{T} < 0$ because heat GOES OUT from your system (water) so has negative sign. Entropy really decreases...but heat goes to the surrounding medium (universe) and my guess is that is going to speed up particles that become more....disordered....increasing the Entropy of the surrounding!!!

Hope it helps! :-)