Question

Explain how water’s entropy can decrease when it freezes without violating the second law of thermodynamics. Specifically, explain what happens to the entropy of its surroundings.

a) The entropy of water decreases due to increased molecular order.
b) The entropy of water increases due to decreased molecular order.
c) The entropy of water remains constant during freezing.
d) The entropy of water is not affected by freezing.

Answer 1

Water's entropy decreases when it freezes, but the second law of thermodynamics is not violated because the heat released into the surroundings increases the surroundings' entropy more than the water's entropy decreases.

Step-by-step explanation:

When water freezes, its entropy decreases because the water molecules arrange into a more ordered, crystal lattice structure. This change seems to conflict with the second law of thermodynamics, which states that the total entropy of an isolated system can never decrease over time. However, this law does not get violated because as the water freezes and releases heat into its surroundings, the surroundings' entropy increases. This increase in entropy of the surroundings more than compensates for the decrease in the water's entropy, thus the overall entropy of the universe increases.

In the formula H₂O(l) → H₂O(s), when water transitions from liquid to solid below its freezing point, although there is a decrease in entropy (ΔS) for the water, the heat given off (ΔH) to the surroundings actually results in an overall increase in the universe's entropy, satisfying the second law.