Final answer:
The pressure of a gas decreases when it moves from a smaller container into a larger space; this is because the molecules have more room to move, resulting in fewer collisions with the container walls and therefore a lower pressure.
Step-by-step explanation:
When gas in an open container moves into a larger empty space, its pressure decreases. This occurs because as the volume of a container is increased, the gas molecules have more space in which to move around.
According to the principles of gas behavior, the frequency of collisions of the molecules with the walls of the container reduces when the volume expands, leading to a lower pressure.
Essentially, if the volume increases while the number of gas molecules and the temperature remain constant, the pressure will decrease in accordance with Boyle's Law.
Physically, what is happening is that the increased volume makes gas molecules more spread out, reducing the number of collisions with the walls of the container, which decreases the pressure exerted by the gas.