If Gas is the only state of matter where you can change the density with pressure, how can you pressurise liquids like water?

Question

I know that solids and liquids can’t change density without changing their molecular structure, so how do things like oil pressure and water pressure exist?

In: Physics

Answer 1

>I know that solids and liquids can’t change density without changing their molecular structure, so how do things like oil pressure and water pressure exist?

Packing and molecular structure are two different things. Think of a loose marble as a molecule. A few in a box is a gas. A compressed gas is the same box with a few more marbles. That same box packed full is a solid or liquid made up of those same marbles/molecules.

Understand that a compressed gas like Nitrogen is N2 when it is compressed to sea level (1 atmosphere or 1 atm), and it is still N2 when it is compressed to 200 times sea level (200 atmospheres or 200 atm.)

It’s just packed more densely at 200 atm than at 1 atm. The box from the above example still has a bunch of loose marbles rattling around in it.

But it can be packed far, far more densely than then simple pressurization. That more dense packing is not done with pressure, but by removing heat from it.

https://en.wikipedia.org/wiki/Liquid_nitrogen

If I really want to compress Nitrogen, I cool it, and then it’s almost as tightly packed as it can be, and becomes a liquid. And it is still the same molecule, N2. Those molecules/marbles are just as tightly packed as they can. If I then pressurize that liquid nitrogen to 200 atm, it will not get more dense, because the marbles/molecules are already as tightly packed as they can be. It can the pressurized to whatever pressure I subject it too, but it does not get more dense.

Density is only loosely related to pressure overall. All solids and liquids are more dense than any gas. Everything can be pressurized, but only gases will change density when pressurized.

Pressurize a gas, and it will get more dense. Pressurize a steel bar, and it will not really do much. That’s why we use steel tanks, to hold compressed gases. Nothing happens to the steel even though it is pressurized, even though that pressure is completely unequal between the inside and outside of the tank.