As the gas warms from the ice-water bath to room temperature, the volume of the syringe increases.
A gas expands when it is heated and condenses when it is cooled. As the flask is warmed from 0ºC to room temperature (25°C), the gas expands inside the flask and slowly moves the syringe plunger. This demonstration demonstrates the relationship between temperature and volume (Charles' Law), and can be used to extrapolate absolute zero.
An NSF funded Project