Why does the boiling point of liquids decrease with decreasing pressure?


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.

The boiling point of a liquid is when its internal pressure meets and exceeds the pressure of the atmosphere above it.

There are two ways of achieving this. The traditional way is you heat up the liquid increasing its internal pressure until it is greater than that of the atmosphere.

The second ways is you can reduce the pressure of the atmosphere (e.g. by putting it in a vacuum) until the atmospheric pressure is less than that of the liquid. This is a not uncommon experiment in HS chemistry classes; you can literally boil away the water while also freezing it.

When liquids boil, they get enough energy to overcome the natural cohesion forces that hold similar elements together (thus you get a bunch of spread out, individual modules, a gas, instead of them loosely bound together, a liquid).

In order to fly off on their own, they not only need to overcome their intrinsic cohesive forces, but also any external forces. Doesn’t matter if you have enough energy to go gas if you don’t have enough energy to actually get away.

Basically, a molecule needs enough energy to do both to successfully escape. The higher the atmospheric pressure, the more energy needed to escape off of the liquid.

Boiling occurs when a particle “jumps” out of a liquid with enough energy to not be bound to the liquid by forces in the liquid (bunch of fluid specific forces). These binding forces are similar in air but less, so it takes less energy to pull that out of the air into the liquid than it does to pull it out of the liquid into the air. (Which is why the temperature to condense a liquid is less than the temperature to boil it).

Pressure holds two things in relationship, temperature and amount of stuff in a space, less stuff, same pressure, higher temperature and all permutations of that you want to make. If you have less pressure at the same temperature, there is less stuff in the air. This is the critical part. This means that there is also less stuff to bounce a particle in the air, which sheds energy from that particle. Less energy taken means it keeps what it has when it leaves the surface of the liquid (not going to bore you with the kinetic energy distribution of a group of particles based on temperature. It is simple enough but its stats and I hate stats).

So when it leaves the surface it is deprived of energy less so when it contacts the surface again, it is less likely to have lost enough energy to be bound again by the liquid. This also means it takes less energy (heat) to get the particle off the liquid because it will lose less in the air, so it needs less to stay in the air, lowering the boiling point.

Can explain more if you want but I hope I simplified it enough without sacrificing the thermodynamics too much.