More Science
The density column we made uses a phenomenon called “salting out” to preserve its layers. When salt is added to the mixture, the ions bond with the water molecules and exclude the slightly less polar alcohol molecules from forming hydrogen bonds with water, making it no longer soluble with water. The alcohol stays separate and ends up as its own layer on top of the column because it has a lower density than oil. Since the salt causes all three liquids to be insoluble with each other, you can mix the column as much as you like—it will always settle into three layers.