how does air pressure affect water boiling/ freezing points
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In a perfect vacuum, air pressure of 0, the water molecules on the surface are only being held down by their weight and attraction to the other water molecules because there’s no air sitting on top of it like a lid.
So, when you heat the water, and the molecules begin to get excited and move around more, those molecules on the surface are free to just jump out of the container like popcorn popping.
Compare that with really high air pressure (normal air pressure at sea level is about 14.7 psi) where there is a blanket of air molecules pushing down on the surface of the water. It takes more heat to create more kinetic energy to enable those surface molecules to jump out past that layer of air sitting on them.
Air pressure has a significant impact on the boiling point – liquids enter the gas phase much more readily at lower pressure.
You can boil water at room temperature with sufficiently low pressure.
It has a much smaller effect on freezing point. Water is a fairly unusual material that expands as it freezes, so increasing the pressure makes it slightly harder to freeze and lowers the freezing point – but you need *a lot* of pressure to notice a difference.
Say I grab you in a bear hug.
Whether or not you escape my bear hug will be influenced by two things:
1. How much energy you can put into struggling
2. How tightly I can squeeze
If you don’t struggle at all then it takes me very little pressure to squeeze you and keep you from escaping, or even to squish you against my chest completely. Similarly, if you can put a lot of energy into struggling then it would take a lot of pressure for me to squeeze you and keep you from breaking free.
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How hard you struggle in this analogy is temperature, or how much kinetic energy a substance has.
How tightly I can squeeze in this analogy is atmospheric pressure.
So, the less atmospheric pressure, the less kinetic energy it takes for a substance to change from solid to liquid, and again from liquid to gas.
Conversely, the greater the atmospheric pressure the more kinetic energy it takes for a phase change to occur.
For a static temperature, as atmospheric pressure increases, a substance will change from gas to liquid to solid.
Easiest explained with boiling point.
When water boils, molecules of water are flying off the surface of the water body. When the air pressure is high, the air in contact with the water is pushing harder on the surface of the water and preventing it from flying away to a greater degree.
The opposite is also true, low pressure means water can fly away easier, thus the boiling point is reduced.
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