Does Liquid in a Vacuum Boil or Freeze?

What happens to a liquid in a cold vacuum? Does it boil or freeze? These animations of liquid nitrogen (LN2) in a vacuum chamber demonstrate the answer: first one, then the other! The top image shows an overview of the process. At standard conditions, liquid nitrogen has a boiling point of 77 Kelvin, about 200 degrees C below room temperature; as a result, LN2 boils at room temperature. As pressure is lowered in the vacuum chamber, LN2’s boiling point also decreases. In response, the boiling becomes more vigorous, as seen in the second row of images. This increased boiling hastens the evaporation of the nitrogen, causing the temperature of the remaining LN2 to drop, the same way sweat evaporating cools our bodies. When the temperature drops low enough, the nitrogen freezes, as seen in the third row of images. This freezing happens so quickly that the nitrogen molecules do not form a crystalline lattice. Instead they are an amorphous solid, like glass. As the residual heat of the metal surface warms the solid nitrogen, the molecules realign into a crystalline lattice, causing the snow-like flakes and transition seen in the last image. Water can also form an amorphous ice if frozen quickly enough. In fact, scientists suspect this to be the most common form of water ice in the interstellar medium. (GIF credit: scientificvisuals; original source: Chef Steps, video; h/t to freshphotons)

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