Gas pressure in a closed system is caused by: resultant force of all combined collisions heat given off by constant collisions rapid diffusion of gas throughout the system compression of the gas

Gas pressure in a closed system is primarily caused by the resultant force of all combined collisions. Gas molecules are in constant, random motion and collide with the walls of their container. Each collision exerts a force on the wall, and the cumulative effect of these numerous collisions results in gas pressure.