You are given an unknown liquid and asked to determine its identity. you can measure only one physical property. you decide to heat the liquid. you record that the liquid turns to vapor at 100°c. this demonstrates which physical property?

The fact that it turns from liquid to vapor, means you're dealing with the boiling point of the substance. If its temperature doesn't vary during the turning of phase, it means the substance is pure. In this case, since it becomes liquid at 100ºC , it might probably be water. 

I hope this helps :)