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It's easy for me to imagine gas pressure on a molecular level. More pressure means that more molecule bounce against the thing you are measuring, or the molecules hit it with more speed or the molecules are heavier.

But I can not imagine how water pressure works on a molecular level. Water is not compressible, so there can not be more water molecules per volume to increase pressure. But if the velocity increases, this should also increase the water temperature. However the bottom of a pool of water is not hotter than the surface. So I just don't understand it.

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  • $\begingroup$ Good question. The reason water is almost incompressible, is because molecules are very tightly packed and there is not much room for them. So you can model it like a ball pit. Do you understand how pressure works in a ball pit? ;-) $\endgroup$ – Ali Jun 3 '18 at 14:41
  • $\begingroup$ "But if the velocity increases, this should also increase the water temperature. However the bottom of a pool of water is not hotter than the surface." I'd suggest shying away from assuming a distinction between pressure and temperature. $\endgroup$ – Nat Jun 3 '18 at 15:59
  • $\begingroup$ @Nat But there is a difference between the two! The bottom of a pool has more pressure but not a higher temperature. And it's not that the temperature rises undetectably slowly as pressure increases, because I can change the temperature without the pressure going through the roof. $\endgroup$ – NounVerber Jun 3 '18 at 16:03
  • $\begingroup$ @Ali No, I don't know how pressure in a ball pit works, if the balls behave like uncompressable molecules. How does a ball "know" that there are a hundred layers of balls above it? $\endgroup$ – NounVerber Jun 3 '18 at 16:04
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The fundamental difference between a liquid and an ideal gas is that in the gas, the molecules fly around free and only occasionally and briefly interact with each other, whereas in a liquid, the molecules are close enough together that they are always within the electromagnetic fields of their neighbors. If two molecules in a liquid move apart a bit, there is a weak electromagnetic attractive force between them. If they move a bit closer together, there arises a repulsive force between them that grows rapidly as the separation decreases. So it doesn't take much compression (but some) for the pressure to increase markedly. Though things at the molecular scale are governed by quantum mechanics, it is fair to imagine a little free-body diagram for a molecule of water which is closest to the "thing you are measuring". There would be a force on the molecule from the "thing", but also forces, on the whole in the opposite direction, from the neighboring molecules.

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