Why does the boiling point of liquids decrease with decreasing pressure?
In: Chemistry
Boiling happens when the kinetic energy (how fast) the molecules are going gets high enough that it overcomes the molecules’ attractions to each other and the molecules can escape on their own. That’s basically the definition of gas.
Outside pressure tends to push the molecules closer together. Most liquids aren’t very compressible so they don’t change size (much) with pressure, but they still transmit pressure through themselves. In order to boil, you need to overcome *both* the molecular attraction and the outside pressure trying to push the molecules back together. Higher pressure means more pressure to overcome to escape, means more kinetic energy needed, means higher temperature.
Think of it like a tightly packed crowd trying to expand out (say, from a street into a square). If there’s no opposition they just flow out and disperse. If there’s a barricade blocking (pressure) them they need to push harder until they can overcome the barrier.
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