Which phase has the lowest entropy? Why? How does a change in phase affect entropy changes in a reaction?

The solid phase has the lowest entropy.

Explanation:

Entropy (S) is the measure of how  spread out or dispersed the energy of a system is among the different possible ways  that system can contain energy. The greater the dispersal, the greater is the entropy.

In a solid, the atoms or molecules  are confined to fixed positions, thus, the dispersal is minimal and the entropy is at its lowest.