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


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

Your premise is faulty.

You can certainly compress solids and liquids. In all the states, molecules will move when compressed.

Well, you can’t. Water pressure is the term used in reference to the object onto which the water is applying force, or pressure.

For example, the water pressure at the bottom of the Marianas trench in enough to crush your average Mini Cooper, but if it’s only 30 feet under, the car won’t collapse. This has to do with the weight force applied onto the object by the water and not the density of the water.

EDIT: sorry, I’m stupid and as someone pointed out to me, the Mini Cooper would have to be an enclosed object to actually crumble in the way I described it. Whoops.

Very simply Gas isn’t the only state of matter that can be compressed / have their density changed.

Liquids can be compressed as well without changing their molecular structure as well as solids.

Just because you get a handful of dirt and compress it into a solid lump of dirt it hasn’t changed the molecular structure of the soil, it’s simply compacted it and thus made it more dense

Solids and liquids do compress, but by much, *much* less than gasses do. For gasses, volume is inversely proportional to pressure: multiply pressure by X, divide volume by X.

For water, on the other hand, you need *lots* more pressure to get a significant volume change. It takes about 3,000 psi (~200 atmospheres!) to compress water by even 1%. So for most practical engineering purposes, you can take liquids to be incompressible and not be very wrong. Some solids are even harder to compress: steel, for example, takes ~230,000 psi to compress by 1%.

Simple answer: you use a pump. You can’t really *compress* a liquid, but you can push it. In most cases you need *flow* more than anything else, and the pressure created is the reaction force from the fluid being pushed by the blades of the impeller.

But, let’s say you have a weird case where you need a liquid at a high pressure (this does happen, sometimes you want to get water really hot without actually boiling it). What you do in that case is leave some space at the top of the tank, and have a compressor squeezing a gas into that space. Whatever pressure the gas is at will be the same in the liquid.