Published: 29 December, 1999

Low pressure can turn gas into solid

Can you turn gas into a solid form?

Yes, you can turn a gas into a solid. Think first about the opposite: turning a solid into a gas. That is a common experience. Under ordinary conditions "dry ice", which is solid carbon dioxide, evaporates directly into a gas without first melting. (That's why it's called "dry".) The evaporation of a solid into a gaseous form is called "sublimation".

It also happens with ordinary ice when it is cold enough. Ice cubes in your freezer gradually shrink because they evaporate into water vapor, and ice on your sidewalk in the winter, if it stays cold enough, slowly evaporates without first melting. Now for your question. That evaporation of a solid into a gas can be reversed and the gas turned directly back into a solid without first becoming liquid. As long as the gas itself is at a low enough pressure it can be turned directly into a solid by cooling it to a low enough temperature. For example, you could start with the gas compressed and then suddenly release the pressure (as when you use a carbon dioxide fire extinguisher). That produces at the same time the low enough pressure AND the low enough temperature for the gas to turn solid without first becoming liquid, and you can actually see the flakes of solid that form."