Hi HN, I remember coming across a post or comment made here some time around 1-3 years ago describing a puzzle described roughly as follows: Consider a sealed chamber with a partition dividing it into two sides, one with Helium gas and the other empty, a vacuum. What happens to the average temperature of the particles in the chamber if the barrier is removed?

Straightforward application of the Ideal Gas law would suggest that the temperature of the gas would decrease since the removal of the barrier would increase the volume occupied. However, the puzzle was thoroughly answered by a wondrously animated/displayed blog illustrating with diagrams how depending on the type of gas, the size of the chambers and interactions between other molecular forces, the temperature could increase or even not change at all! I am probably inaccurately recalling to some degree, for example: I can't guarantee that the gas mentioned in the puzzle was specifically He (it may have been left ambiguous, along with the relative sizes of the sub-chambers) however I do remember a surprising tripartite conclusion to a seemingly straightforward application of the law accompanied by glorious (maybe even interactive) diagrams. I originally found the content through this lovely website and would like to pose the question: Does anyone know what the heck I may be talking about?!?

After a few hours of searching via google/algolia, I have resigned the link lost to me. To make matters more complicated, I believe the title of the solution blog or relevant post was ambiguously puzzling/curious and not overtly physics-y which is unlikely to appear amidst queries for 'gas tube temperature puzzle' and the like... If anyone could help reunite me with this puzzle or elegant solution I would be grateful, and I'm sure others would appreciate the surprising conclusion as well.