|MadSci Network: Physics|
The "blue" vapor you see is actually an "aerosol" effect upon opening the can. In a typical "12oz" can there is approximately 0.25oz of space above the beverage. This volume is filled with C02 at approximately 45psig pressure and is saturated with "beverage vapor". (The actual pressure will vary depending upon the beverage; fruit flavored drinks are around 20psig, club soda is 55psig.)
The Ideal Gas Law (pV = nRT), can be used to describe the relationship between temperature, pressure, mass and volume under equilibrium or quasi-equilibrium conditions, but this is not satisfied in the case of opening the beverage can. There may be a slight drop in temperature as you open the can, but you would need extremely sensitive instrumentation to actually measure it.
Depending upon the age of the students, I would suggest one of the following demonstrations:
Alcoa Technical Center
Try the links in the MadSci Library for more information on Physics.