I've had boiling explained to me in two ways, and I'm having trouble understanding how they connect.

The first way is that a liquid is held together by the intermolecular forces, and boiling involves breaking those bonds/interactions. This explains why, for instance, the boiling point of water is higher than that of methane, but not why boiling point is pressure dependent.

The second way is in terms of vapor pressure. Boiling occurs when bubbles form, and in order for a bubble to form and not be crushed, its internal pressure must be equal to the atmospheric pressure. From this view, the boiling point is the temperature at which the vapor pressure equals the atmospheric pressure. This explains why boiling point drops as pressure drops.

My issue is understanding how these coexist. The way I think of it, the strength of the intermolecular interaction should be the same no matter the pressure, so the temperature to break those interactions should be constant. Also, you can boil a liquid just by lowering the pressure, but I don't see how this affects the intermolecular interactions in any way.


1 Answer 1


Consider boiling as the rapid escape of molecules.

In slow evaporation and in rapid boiling, only those molecules with sufficient kinetic energy are able to leave the liquid. As each "fast" molecule leaves, it subtracts its high KE from the average KE pool of the liquid, i.e. cooling it.

For evaporation to take place, there has to be a chance for molecules to escape and not to be replaced by those in the gas above the liquid. In fact, if the pressure is high (below the critical temperature), then the gas can condense to the liquid, faster than the liquid can evaporate to the gas.

For boiling to take place, there is the further restriction that localized temperature rise takes place faster than in the main body of the liquid, so that specific locations overheat, e.g. where heat flux is highest. Where pressure is a factor, then boiling may be prevented until the pressure is relieved, as in "black smokers". The water exiting the vent may be at 200 C or more, but does not boil unless the critical temperature is exceeded.


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