Explain why changes of state are physical changes.

Physical changes are those that have a change in "characteristics" therefore state change is when the state (solid, liquid, gas) are changed. 
In your answer you can put because state change is when the substance changes to a solid, liquid, or gas. 
sorry if its confusing... however chemical changes are when there is a rection between two interactions of substances and there is gas formed, along with heat, and is difficult to reverse.