Why Does Density Affect the Density of its Container?

[Ryan Reed]

I've never been able to figure this out. I can understand when you put oil in water that one floats on top of the other, but why does it still work if I put it into a balloon? Something that keeps it from directly interacting with the medium it's in?

 

[mfb]

Why should oil and water behave differently in a balloon (compared to what, a bucket)? The heavier medium feels a greater force per volume to go downwards, so energy can be released by the separation.

 

[Ryan Reed]

Sorry I didn't clarify, I meant in a atomic level, what is happening that causes buoyancy?

 

[mfb]

The force differences. It doesn't matter at which scale you look at it.

 

[sophiecentaur]

Forgetting about floating and sinking, which are macroscopic issues, the definition of density is the same on any scale.
Density = Mass per unit volume
You get the same answer if you are dealing with a bucket sized sample or a single, whole molecule. Looking inside a molecule, the Mass is not distributed uniformly so the dense bits will be the nuclei and there will be ( mostly) empty space in between occupied by some electrons.
When your substances are fluids, on the ground, the more dense substance will fall down and displace the less dense. That is a universal rule unless there is some chemical reaction between the two substances.