As gases cool, their velocities slow, and the potential energy that draws them together becomes greater relative to the kinetic energy the pulls them apart.  This is demonstrated as a large balloon is made to fit into a small beaker.  The balloon is filled with ordinary air (it was, in fact, blown up in front of the audience), and the beaker contains liquid nitrogen (which boils at -195.79 degrees Celsius, or -320.42 degrees Fahrenheit).  As the bottom of the balloon touches the liquid nitrogen, any gas near the balloon begins to contract.  The pressure in the balloon decreases, and the volume of the balloon begins to decrease as well, until the entire balloon is smaller than the beaker.  If allowed to contract slowly, it will resume its original shape, but one must be careful not to bump the balloon as the rubber is brittle and needs time to warm up to regain its elastic properties.  

This demonstration is based on well known properties of gases as they cool and properties of liquid nitrogen.  As with any demonstration, be sure to follow the directions very carefully and observe all applicable safety precautions.  This web site does not purport to be a site designed to convey directions for chemical demonstrations, and is not liable for any injuries or damages sustained by those who would attempt to re-create this demonstration without proper training, supervision or instructions. 

Concepts:   Air Pressure, Contraction of Gases on Cooling

Time Required:   15 minutes

Chemicals Needed:  None  (Al pop can)