The situation is a thermally insulated container divided into two compartments, which are separated by a diathermic piston. Initially, the left compartment has a volume (V1) of 2L, and a temperature of 400K, and the right has a volume (V2) of 4L and a temperature of 200K. Both contain one mole of ideal gas each. The piston is first clamped, and then released. The question states that work associated with the movement of the piston may be neglected. The question asks to describe the graph of volume and pressure versus time for this spontaneous change, and these were the correct solutions (pictures below).
I calculated that the final volumes are 3L, and the final pressures are 100 atm in both compartments. (using the fact that that temperatures equalise to 300K while the piston cannot move, final pressures of both compartments must be equal, and number of moles in each compartment must remain the same)
What I do not understand is the manner in which this final 3L is reached. Specifically, I do not understand why V1 rises to a peak and then reduces and why V2 decreases to a minimum and then increases to the final value.
I've researched any other similar piston questions, but could not find an explanation for it. Could anyone please help with this? (I am in 12th grade, and this question is from an exam meant to be given by 12th graders)