An ideal gas at constant temperature is confined in an airtight cylinder, and a piston pushes down on the gas in the cylinder. assuming the gas behaves as an ideal gas during the compression, what effect will this have on the pressure of the ideal gas?

Pushing down a piston in an airtight cylinder would tend to increase the pressure in the system since the particle collision and collisions with the walls are now more frequent. Also, pushing further, the gas would undergo a phase change from gas to a liquid.