Final answer:
Opening the valve in the side of the glass bell jar causes the air pressure inside to decrease due to the escape of air, which reduces the number of molecules inside the fixed volume.
Step-by-step explanation:
When the valve in the side of the glass bell jar is opened, the air pressure inside the bell jar decreases. This is because when the valve is opened, air inside the bell jar can escape, reducing the number of air molecules within the fixed volume of the bell jar. The reduction in the number of molecules leads to less frequent collisions with the walls of the bell jar, thereby lowering the pressure inside the jar.
According to principles of physics, particularly Boyle's law, which relates the pressure of a gas to its volume when temperature is constant, a decrease in the number of gas molecules in a fixed volume lowers the pressure. This principle is also demonstrated in scenarios such as opening a bottle of sparkling water, where releasing the pressure allows carbon dioxide to form bubbles, and the water eventually goes flat as the gas escapes and the pressure equalizes with the atmospheric pressure.