Water freezing is an example of _________ .

When water freezes, it converts from liquid water to solid water (also known as ice). Thus water freezing is an example of a phase transition. Change in phase is a physical change, since the molecules remain the same and there is no change in the chemical composition of the material. Similarly, the boiling of water to produce steam is an example of a phase transition and a physical change. In comparison, a change in chemical composition indicates a chemical change.

Phase transition is either an endothermic reaction or an exothermic reaction. When water freezes, it releases some energy and hence freezing is an exothermic reaction. More specifically, 80 calories of heat is lost to the environment when 1 gram of water freezes. This is also the reason why we have to heat the ice to melt it and thus, melting is an endothermic reaction. 

Hope this helps.