# When is total pressure not conserved in a system?

I came across a problem that involved two compartments that are separated by a movable, adiabatic wall. As the wall moves, the pressure is not conserved- rather total pressure decreases- assuming this is an ideal gas. How is this possible? Doesn't the second law of thermodynamics tell us that total pressure should be conserved?

• No, the 2nd law of thermodynamics states that the entropy of an isolated system can never decrease with time. There is no such thing as a law of "conservation of pressure". – Samuel Weir Sep 12 '19 at 17:33
• Why wouldn't it be conserved though in this case? If its a closed system how does pressure decrease with movement of the wall? – Linda Sep 12 '19 at 17:35
• If I correctly understand the system that you are describing, as the wall is moved the pressure in one of the compartments should increase and the pressure in the other compartment should decrease. – Samuel Weir Sep 12 '19 at 17:40
• So the way it is presented here, originally the two compartments have pressure of 1 and 4 atm and then once the wall shifts the two compartments have pressures of 1.75 each and thus it decreases overall – Linda Sep 12 '19 at 17:44
• Why is it not possible for the volume to be 2.5 each? Why does it decrease overall? – Linda Sep 12 '19 at 17:45