The Earth is one of those extremely rare, special places in the Universe where water can exist, stably, as a liquid. So much of it exists here on Earth, that if you were to add up all the oceans on Earth together, it would weigh more than 10^18 tonnes, more massive than the biggest asteroid ever, and about as massive as Pluto's giant moon, Charon.

But water only has a very small window in which it can be a liquid. For instance, if you took some warm water up to a very high elevation, it would start to boil, and become a gas! The higher up you took it, the lower and lower your boiling point would be.

Why? Because higher altitudes on Earth mean lower pressure. If there's not enough force pressing the water into a liquid phase, then there's no force binding the water molecules together. If you simply allow them to diffuse away, they will. And that's the definition of a gas, which is what you'll wind up with.

On the other hand, water has no business being a liquid at low temperatures, either. You can see -- from this diagram below -- that if you start with liquid water, you can turn it into a gas by lowering the pressure, but you can also turn it into a solid by lowering the temperature.

So my question is this:

If you took a glass of water into outer space, would the water freeze or would the water boil?

This is a question that seems awfully tough, because in addition to knowing about water:

We also need to know about outer space. Space is a lot of things: cold, dark, and empty come to mind right away. And they come to mind, pretty much, as soon as you leave the Earth.

Well, the temperature of space is, at its coldest, just the temperature of the leftover glow from the Big Bang. This radiation, known as the Cosmic Microwave Background, bathes the entire Universe in a temperature of only 2.7 Kelvin. That's less than 3 degrees above absolute zero, or -455 degrees Fahrenheit! But there's also -- literally -- no pressure in space. So, what happens? Who wins? Does the water freeze or boil?

Oddly enough, the answer is first one, and then the other! It turns out that having a pressure vacuum will cause the water to boil almost instantly. In other words, the effect of boiling is much, much faster than the effect of freezing.

But the story doesn't end there. Once the water has boiled, we now have some isolated water molecules in a gaseous state, but a very, very cold environment! These tiny water vapor droplets now immediately freeze (or, technically, desublimate), and become ice crystals.

We've observed this before. According to astronaut observations, where they've observed their urine get expelled from the ship:

When the astronauts take a leak while on a mission and expel the result into space, it boils violently. The vapor then passes immediately into the solid state (a process known as desublimation), and you end up with a cloud of very fine crystals of frozen urine.

Sounds like it would be a fantastic thing to watch, doesn't it? Well, we've done almost the same thing on Earth. What happens if you take boiling water and, on a very, very cold day, throw it up into the air?

The water finishes boiling and becomes a gas, the gas freezes (or desublimates), and ice crystals -- a.k.a. snow -- results! And that's what happens to water in space. Oh, to be able to try this at home...