If melting of ice into water increases entropy, then does its freezing due to naturally occurring cold temperatures decrease the entropy? And does the entropy of the ice decrease or that of the whole Universe too? If it does not in case of the Universe, then why?
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The second law of thermodynamics states that the entropy of an isolated system never decreases.
If you put water in a container in deep space it will cool down and freeze. The entropy of the container and water will decrease.
The reason the container of water does not break the second law is that it is not an isolated system. In an isolated system, there is no exchange of energy and matter with anything else. The container does lose energy that is radiated out as infrared light so not an isolated system. You can approximate the container and what is radiated out as a isolated system,
So the question is what is the change in entropy of the container and what is radiating out into space? The answer, in this case, will be that it has not decreased, it will in fact increase.
If you put water in a cold enviorment it gets cooled down by heating out the sounding area. The temperature will equalize, The entropy when the temperature equalized is higher than when the water was warmer and surrounding was cooler.
So entropy can and do locally decrease but at the same time, it will increase more in the surrounding that it thermodynamically interact with. A local decrease it not against the law of thermodynamics.
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