1. The problem statement, all variables and given/known data a) In general, can we determine the identity of a pure substance based only on its density? b) During the summer the temperature of water is warmer at the surface of a lake than in depth. Disregarding the fact that the lake is heated by the sun light, explain this phenomenon. 2. Relevant equations None 3. The attempt at a solution a) Yes because all pure substances have their own density. Density for all pure substances is different. b) I believe that this phenomenon is called Thermocline. I'm not sure if these answers are correct, if anyone can shine some light, it would be great. It's my first day in college, and I'm having trouble. Thanks :D.