How does density work on a molecular level?

Question

Assume two liquids of different density stacked on top of each other. Or, assume a ballon filled helium gas, in our atmosphere.

1. Why does the lighter fluid “go up”? How does this work out on a molecular level?
2. What would happen when these fluids in free fall, I.E not under the influence of gravity?

In: 8

Answer 1

Denser liquid = more mass per volume, = gravity pulls it down harder than the less dense liquid. Same reason a rock sinks in water. On a molecular level, molecules might be closer together on average, or just bigger individually