In an oil spill, why doesn't the oil mix with the sea water?
Because water is a highly polar solvent........
.........And oil is a highly non-polar solute.....The difference in polarity between the two solvents results in separation of the two phases; i.e. they are immiscible.
The oil is LESS DENSE than the water, and thus it floats on the water, and should this occur with an oil spill, e.g. the Exxon Valdez, this has disastrous consequences for aquatic flora and fauna (and birdies). We see the same phenomenon when we do a phase extraction........
In the illustration, which is the LESS dense layer? Normally, you do not cap the separating funnel when it is resting on a stand. Why not?