# eli5 how aerated water has less or no buoyancy at all

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cant wrap my head around it rn

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When considering if something will float in a fluid the relevant factor is their comparative density. A boat floats on water because its overall density is less than that of an equivalent volume of water; the volume of their hull under the water exactly equals the volume of water which would weigh as much as the boat.

If for example you had a very dense fluid such as mercury then relatively dense things can float in it, like solid iron! Things which have a very low density can also float in relatively low density things, like a helium balloon can float in the air. Even lowering the density of air a small amount such as by heating it will cause it to rise relative to the other air.

With aerated water you are taking a volume of water and lowering its density by filling it will small bubbles of air. A boat which can float on water may no longer float on water full of those small bubbles as it lowers the average density below that of the boat’s hull.

What you’re thinking of as “water” is actually a mix of air and water. Things that float on water won’t float on air, so they sink.

When you put air into water you break apart the molecules slightly. If you ‘zoom in’ molecular level, molecules are rubbing and slipping among one another, like oiled up guests at some Vegas nightclub. If you have a lot of these molcules and you plop a solid in it that has air, its own fluids, etc, the molecules ‘displace’ – they move aside. But since fluid doesn’t easily compress, they exert a force back the body. It is why boats sink down into the water under weight and don’t sit on the surface. Some things can do that, things that are light enough to not break the surface tension of the water.

When you put air molecules in water you are forcing the fluid molecules apart a little bit, which gives them an ability to compress and reduces the ability to exert force back on the solid body that entered it.

Water is more dense than air. If you add bubbles, then there’s less water and more air per volume area. This makes the volume area less dense. Higher density things sink, so things that were less dense than water and floated (e.g. boats) are now more dense than the aerated water. These things then go blub blub blub (they sink).

Buoyancy is based on relative density.

The more dense object sinks into the less dense one (or the less dense one rises to the top,)

When water is full of air bubble it is less dense (it is also not an even mix)

Bubbles rise to the top, and anything heavier than air tends to fall to a region of similar buoyancy.