Would a Glass of Water Freeze or Boil in Space?

Boiling point of water in a vacuum

 Haitong Yu / Getty Images 

Here's a question for you to ponder: Would a glass of water freeze or boil in space? On the one hand, you may think space is very cold, well below the freezing point of water. On the other hand, space is a vacuum, so you would expect the low pressure would cause the water to boil into vapor. Which happens first? What is the boiling point of water in a vacuum, anyway?

Urinating in Space

As it turns out, the answer to this question is known.

When astronauts urinate in space and release the contents, the urine rapidly boils into vapor, which immediately desublimates or crystallizes directly from the gas to solid phase into tiny urine crystals. Urine isn't completely water, but you'd expect the same process to occur with a glass of water as with astronaut waste.

How It Works

Space isn't actually cold because the temperature is a measure of the movement of molecules. If you don't have matter, as in a vacuum, you don't have temperature. The heat imparted to the glass of water would depend on whether it was in sunlight, in contact with another surface or out on its own in the dark. In deep space, the temperature of an object would be around -460°F or 3K, which is extremely cold. On the other hand, polished aluminum in full sunlight has been known to reach 850°F. That's quite a temperature difference!

However, it doesn't matter much when the pressure is nearly a vacuum.

Think about water on Earth. Water boils more readily on a mountaintop than at sea level. In fact, you could drink a cup of boiling water on some mountains and not get burned! In the lab, you can make water boil at room temperature simply by applying a partial vacuum to it. That's what you would expect to happen in space.

See Water Boil at Room Temperature

While it's impractical to visit space to see the water boil, you can see the effect without leaving the comfort of your home or classroom. All you need is a syringe and water. You can get a syringe at any pharmacy (no needle necessary) or many labs have them, too. 

  1. Suck a small amount of water into the syringe. You just need enough to see it -- don't fill the syringe all the way.
  2. Put your finger over the opening of the syringe to seal it. If you're worried about hurting your finger, you can cover the opening with a piece of plastic.
  3. While watching the water, pull back on the syringe as quickly as you can. Did you see the water boil?

Boiling Point of Water in a Vacuum

Even space isn't an absolute vacuum, although it's pretty close. This chart shows boiling points (temperatures) of water at different vacuum levels. The first value is for sea level and then at decreasing pressure levels.

Boiling Points of Water at Different Vacuum Levels
Temperature °FTemperature °CPressure (PSIA)