Final answer:
The gas pressure decreased due to a reduction in the number of gas particles in the sealed container, resulting in fewer collisions and thus lower pressure.
Step-by-step explanation:
The student is asking about the relationship between the number of gas particles and gas pressure in a sealed container with fixed volume and temperature. According to Boyle's law, at a constant temperature, the pressure of a gas is inversely proportional to its volume. However, since in this case the volume and temperature are constant, the decrease in pressure from 50 kPa to 25 kPa must be caused by a reduction in the number of gas particles. Gas pressure is created by particles colliding with the walls of their container. If the number of particles is halved, the frequency of collisions is also halved, leading to a decrease in pressure by the same factor. Therefore, the change that must have occurred in the cylinder is the removal of gas particles. This explains the reduced pressure, as there are fewer particles to exert force upon the walls of the container.