@chemteacherphil: If you want to convert a gas to a solid it requires removing a lot of energy from the gas. There are many ways to do this, but a super fast way involves rapid expansion of a compressed gas. In order for the compressed gas to expand, it has to push other atmospheric gas particles out of the way. And this requires energy. Since the process is so quick, the work required comes from the internal energy of the expanding gas, resulting in the rapid cooling of the gas molecules. In this case, the stable form of carbon dioxide at atmospheric pressure is a solid, so the gas forms little bits of dry ice “snow” as it expands. The wooden form you see me using captures these little bits and squishes them into a solid brick! How cool is that! #chemistry #chemteacherphil