Why oil floats on water

Oil floats on water because a typical oil is less dense, so the heavier water slides underneath and buoys it up. The two also refuse to dissolve into one liquid, which is why the oil stays a separate layer instead of vanishing.

Floating is not proof that the liquids push each other away like magnets, and the layer is not a skin the oil grows on purpose. Density decides which one ends up on top, while the failure to mix decides that you can see a boundary at all.

The mechanism is more particular than the slogan. Water molecules pull on one another through hydrogen bonds, a tight electrical sticking of their charged ends. Oil molecules are nonpolar, with their charge spread more evenly, so they are not invited into that network and they stay with each other.

Follow it one step further. Inside each layer, weight and pressure still follow the old buoyancy rule. A parcel of oil weighs less than the parcel of water it displaces, so the net force on it is upward until the oil is all on top.

The case worth keeping in mind is specific. A spoon of cooking oil poured into a glass of water beads, rises, and spreads into a lens at the surface. Salad dressing left alone in the jar does the same thing, and shaking the jar only postpones the split.

That case has a boundary. Not every liquid that refuses to mix with water is lighter than water, and some dense organic liquids sink into a layer of their own on the bottom. Ordinary kitchen oils and most crude oils happen to be the floating kind, which is a fact about their density.