Seven-Layer Density Column: Stacking Liquids

Oil and water don’t mix. Everyone knows that. What is the scientific reason behind why don’t they mix?

Answer: Oil and water don’t mix because of how their molecules are constructed. Water is what is known as a polar molecule. A water molecule is shaped like a V, with an oxygen atom at the bottom point of the V and a hydrogen atom on each of the two top ends. However, there is unequal sharing of electrons between the hydrogen and oxygen atoms. This means that the bottom of the molecule has a negative electrical charge, while the top car- ries a positive charge.

Vegetable oil, on the other hand, is a nonpolar molecule made of long chains of hydrocarbons—strings of carbon atoms bonded to hydrogen atoms. Unlike the water molecule, there is equal sharing of electrons between the carbon and hydrogen atoms. This means that the electrical charges of the atoms are not separated, so the molecules don’t have opposite positive and negative ends.

If you were to think of molecules like groups of people, the polar molecules hang out with other polar molecules, and the nonpolar molecules with other nonpolar molecules. This brings us back to the reason why oil and water don’t mix. Water is a polar molecule, and it just doesn’t hang out with nonpolar molecules like oil. Scientists say that oil and water are immiscible.

The adage “like dissolves like” will help you remember what will mix with what. Salt and water mix because both molecules are polar—like dissolves like. It’s also easy to mix vegetable oil and olive oil, or motor oil and peanut oil . . . but that’s gross.

You can do this experiment at home using liquids you find around the house. Some of my favorites are honey, Karo syrup, Dawn dish soap, water, rubbing alcohol and vegetable oil. Experiment with the different densities of the liquids to determine which ones will float on top of the others. Hint: start by figuring out which liquid has the highest density.