Molecular Monday: When Water Boils and Freezes at the Same Time

Welcome to Molecular Mondays! Every other Monday, we examine the atoms and molecules that serve as the building blocks of our universe, both in reality and in science fiction.

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At some point, you may have heard in a science documentary or something that in the vacuum of space, liquid water boils and freezes at the same time. So what the heck does that mean?

Maybe it means water behaves something like this:

The chemical in that video is called cyclohexane, but it could just as easily have been any other chemical, including water, that has been brought to its triple point.

In a previous Molecular Monday post, we talked about how water’s freezing and boiling temperatures change as you raise or lower the pressure. At a certain point, when the pressure is low enough, the freezing and boiling temperatures are the same, causing water to freak out.

Since water is trying to be all three states of matter simultaneously, we call this the triple point.

The temperature and pressure in space are well below water’s triple point, precluding any possibility of a liquid state. So if you put liquid water in space, it starts changing into something else. Anything else.  As rapidly as possible.

According to astronauts who’ve watched this happen (i.e.: astronauts who’ve looked out the window after using the space toilet), liquids tend to turn into vapor first, and then the vapor turns into ice crystals. So all those science documentaries should really say that liquid water in space boils then freezes, or rather it boils then desublimates. Although given that these changes occur in a matter of seconds, I won’t be too nitpicky.