Final answer:
When the valve in the side of the glass bell jar is opened, the gas inside the jar becomes evenly distributed with the gas in the second flask due to diffusion.
Step-by-step explanation:
When the valve in the side of the glass bell jar is opened, the gas inside the jar spontaneously becomes evenly distributed between the jar and the second flask containing a vacuum, due to the principle of diffusion. Diffusion is the process by which molecules move from areas of high concentration to areas of low concentration until they are evenly distributed. In this case, the gas molecules move from the region of higher concentration inside the jar to the region of lower concentration in the empty flask.