This was done to illustrate that a. Gases condense into liquids at very low temperature. The van der Waals equation of state gives a more accurate value for the volume V at low temperature. The volume of a fixed number of moles of gas at constant atmospheric pressure will decrease as T decreases . Rubber freezes and breaks at low temperature.
Warm air has higher kinetic energy and thus is more buoyant.
There are fewer molecular aggregates in warm air. A phase separation occurs because of the temperature difference. The ideal gas law predicts that warm air is less dense than cold air at the same pressure. He and other noble gases in the atmosphere concentrate in warmer air. The gas is expanded in a 400 liter glass flask at 35 C, just above room temperature.