There is a silly detail about the interpretation of the Van der Waals (VDW) equation that I cannot fully understand. Say we have the Van der Waals equation for one mole: (P + a / V^2 ) (V - b) = RT The usual interpretation is that if you start from the ideal gas law PV=RT, you have to "decrease" the volume to take into account the finite size of molecules, so you replace "V" by "V-b". The attractive forces also reduce the pressure, so you... replace "p" by "p+a/V^2 " ? Why not "p-a/V^2 " ? Why if both pressure and volume are reduced, you subtract a quantity to volume but add a quantity to pressure? I know the equation is correct as it reproduces experimental results within its domain of applicability, but I would like an intuitive explanation for this. Thanks!