this is my first post here. I watched this video https://www.youtube.com/watch?v=h5wQoA15OnQ and at 51 seconds it describes Bernoulli's effect of a drop in pressure resulting from an increased velocity (when the liquid enters the smaller capillary tubes). Then it states that the refrigerant (the liquid) boils because of this drop in pressure which lowered the boiling point AND there is a resulting temperature drop in the liquid.
Ok so when a liquid boils due to a drop in pressure (like in the throttling device of a refrigerator) why is there a loss of heat, specifically how does the liquid lose its own energy upon evaporation? I'm looking for an explanation that is deep and explains the mechanism from a molecular level.
When the liquid evaporates my understanding is that there is a gain in kinetic energy of the molecules that are escaping and because this energy was present to begin with (because no energy was added) there must then be a loss in the kinetic energy of the molecules in the liquid so the temperature OF THE LIQUID drops.
Ok maybe i just answered my own question but does this then mean that a gas of a fluid has a lower heat capacity than its liquid counterpart, which would be why the gas form of the refrigerant can get rid of heat (to the external environment (in this case away from the refrigerators insulated inside)) more efficiently, or maybe this isn't the case but rather the heat is now more concentrated (is mostly in the gaseous portion of the fluid) and so it can then be released more efficiently…
I'm just unclear on exactly how there is heat loss from a liquid boiling due to a drop in pressure.
Best Answer
Any form of evaporation will cool a liquid, whether it's evaporation due to boiling or evaporation at temperatures below the boiling point.
The temperature of a liquid is related to the average energy of the molecules that make it up. However at any moment in time the energies of the molecules are not all the same - some will have less energy that the average and some will have more energy than the average. The energies of the molecules will follow something like a Boltzmann distribution.
It's always the molecules with the higher energies that boil first, obviously because they are the ones with enough energy to escape from the liquid into the gas. But if we remove the molecules with the highest energies then this must reduce the average energy of the remaining molecules, and therefore the temperature falls.
That's why any form of evaporation cools a liquid - because it preferentially removes the molecules with the highest energies.