A pan of liquid water freezes when you place it outside on a cold day. Liquid water has greater molecular disorder than ice does. Is the freezing process, then, an exception to the law of entropy?

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Physics

Azaria Holt

20 May, 18:46

A pan of liquid water freezes when you place it outside on a cold day. Liquid water has greater molecular disorder than ice does. Is the freezing process, then, an exception to the law of entropy?

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Jenna Benjamin · Answer 1

No.

Explanation:

The freezing process is not an exception, simply because the pan of liquid water cannot be considered an isolated system.

In fact, the law of entropy holds for isolated systems only: the entropy of an isolated system can never decrease. However, in this case the pan of liquid water is not an isolated system, since it exchanges energy with the surrounding. In particular, it gives thermal energy away to the environment, so its temperature decreases and it freezes: the entropy of the pan of water decreases, but the total entropy of the system (pan of water + environment) does not decrease.