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Why does water boil in a vacuum?

Basically in a vacuum there is no pressure. This is important for liquids to remain in a liquid state and the temperature at which they start to boil drops. Water could boil at room temperature under a vacuum. It is crazy but by changing waters structure by applying the vacuum it drops it's boiling point.


As long as the temperature of an object is above absolute zero the molecules it consists of are in motion. Heat energy is random motion of the particles that matter consists of. Temperature is proportional to the kinetic energy of the average particle in matter. The kinetic energy of the particles in an object are unevenly distributed. Some particles have a lot of energy, others have very little and most are close to the average. 

Suppose you have an open vessel filled with water at 1 bar and at 20 C. The quantity of the liquid does not visibly change but some of the water molecules at the surface will always have enough kinetic energy to escape the liquid and become airborne. That process is called evaporation. The process is slowed down by air molecules that exert pressure against the liquid surface (1,013 bar at sea level) by flying against the water surface and colliding into the water molecules trying to escape from the water surface. In a vacuum, there is no gas and thus no gas pressure. When you lower the air pressure, the escaping molecules are hindered less and less by air molecules flying toward them and colliding into them. In a vacuum, there is no resistance from air above the water surface at all. The evaporation quickens and pretty soon there is no liquid water left.

In space, where there is no gravity, either, the evaporation would be even faster because there would be no gravity pulling the water molecules against the bottom of the can at all.


In a vacuum, there is no pressure. This is critical for most liquids to remain in a liquid state because with no pressure, the temperature at which they start to boil drops. Water boils when there is no pressure (and as morbid as it sounds, so does blood

Yes, we do tend to think of water existing as a solid below zero degrees, as a liquid up to 100 degrees and then boil into steam. But, in fact, those are three different states that water can take at any temperature. It's just that, energetically, it prefers to be in particular states at different temperatures.

For example, you can have ice at 4 degrees C and it will be melting, but it hasn't melted yet despite being somewhat warmer than the nominal temperature we expect it to melt at.

Now, the states that water likes to be in at different temperatures change depending what the pressure is. So those temperatures of zero degrees C and 100 degrees C are only what you have at atmospheric pressure - the pressure of the air around us.

But if you take that air away, then, because you haven't got the pressure of the air pushing in on it, water molecules inside that material will naturally want a bit more spaced apart.

That means they're more likely to want to be a gas or liquid than a solid. And so ice melts, and water boils, at a lower temperature