Gas pressure is primarily caused by what phenomenon?

Gas pressure is primarily caused by gas molecules colliding with surfaces. When gas molecules move, they are in constant random motion, traveling at high speeds. As they collide with the walls of their container or any surfaces they encounter, they exert a force on those surfaces. The collective effect of countless collisions between gas molecules and surfaces leads to the pressure that we measure.

This relationship between molecular motion and pressure is fundamental to gas behavior and can be described by the kinetic molecular theory. According to this theory, the pressure of a gas is proportional to the number of collisions per unit area per time and the average force exerted by those collisions.

In contrast, while gas molecules do expand and can condense under certain conditions, these phenomena are more related to changes in volume and state rather than directly causing pressure. The act of changing states is also not directly related to pressure; rather, it involves energy changes and molecular interactions under different conditions. Therefore, the primary cause of gas pressure remains the collisions of gas molecules with surfaces.