Question

Does the following reaction show an increase or decrease in entropy? H2O(l) ---> H2O(s)

Answer 1

The entropy change is negative - a decrease in entropy.

Entropy is associated with the amount of disorder or number of degrees of freedom. In the liquid state the water molecules are able to move around randomly and so are very disordered (although not as disordered as in the gaseous state, when it is steam).

When we freeze water to turn it into a solid, we remove the degrees of freedom associated with the water molecules moving around. The ice is more ordered than water, so has a lower entropy. The entropy change is therefore negative.