Why does water freezing not violate the second law of thermodynamics? (Hint: Think in terms of a system)
The entropy of the water decreases when it freezes and goes into a more ordered crystaline state. However, heat is released to the environment in the process. If the surrounding envrinment is also at 0 C (273 K), the heat gained by the environment equals the heat lost by the ice, so there is no net change in system entropy. If the environment is colder than 0 C while the ice freezes, the environment gains more entropy that the ice loses, because
Q/273 < Q/Tenvironment. Net system entropy increases, which is consistent with the second law.