Why does the boiling point of liquids decrease with decreasing pressure?
In: Chemistry
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.
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