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.

>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.


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.

Most of the water pressure you experience in your home comes from water towers. Water is pumped up into the tower and gravity pulls down on the water providing pressure at your sink.

Or if you fill a tank with water then pump air into the tank the air acts as a spring putting pressure on the water.

Pressure is just force per unit of area, some answers pointed out liquids and solids have some degree of compressibility but it’s irrelevant, an hypotetical incompressible liquid can still be under pressure simply because you can apply forces to it.

If you have a tank of water, the liquid at the top of the tank has to support the weight of all the air above it, and the liquid at the bottom of the tank has to support that plus the weight of all the water on top of it which can be significant if the tank is tall, meaning that liquid will be under a high pressure. And since the liquid transmits those forces in all directions the sides of the tank near the bottom will also have to resist the high pressure.

Solids and liquids are pretty much incompressible (they are compressible but they need crazy amounts of force to do so).

Oil for example is used to transfer forces thanks to it being incompressible. For example in the brake system of cars, oil is used to transfer the force from the brake pedal to the brake pads.

When you hear that the oil pressure is x bar what it really means is that the oil is applying x amount of force per unit of surface on the brake pads.

When mechanics change brake fluid on cars they need to bleed the system so that any air bubble that got in the system gets pushed out. If any air is in your brake system, the force you apply on the brake pedal is used to compress those air bubbles (because air is a gas and is therefore compressible) and not to push the brake pads.

Think of the molecular forces like springs. Gas is a soft spring, water is a stiff spring, steel is a really stiff spring. 10 lbs of force is 10 lbs regardless which spring, the difference is just how much they move. Similar with pressure, if you put a liquid and gas and steel block in a tank and pressurize the gas, all 3 will be at roughly the same pressure, but the gas will change volume a lot, the liquid a little, and the steel the least, but they all do compress and they all “feel” the same pressure

You can pressurize water. it even pressurizes itself under its own weight. It just doesn’t change density.

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