Why does oil float to the top in liquid if you try to mix it with the liquid?

Asked by: Rachel Ginsberg

Answer

I think there are 2 questions here. The first is why oil and water don't mix. The second is why oil rises, as opposed to sinking or staying in place.

First question. There is an old saying in chemistry that like dissolves like. What this means is that a substance tends to dissolve in another substance if the molecules of the 2 substances have similar electric dipole moments. Think of an electric dipole like you think of a magnet. The magnet has a north and south pole, and south end of one magnet is attracted to the north end of another magnet, and vice versa. The electric dipole has a positively charged end and a negatively charged end. The magnitude of the positive charge can be greater than that of the negative charge, or vice versa. The difference between the magnitudes of the 2 charges and the distance between them determines the moment or strength of the dipole. In general, dipoles with similar strengths dissolve in each other more readily than dipoles with very different strengths. Oil (as in hydrocarbon-based oils) and water have very different dipole moments, so oil and water do not readily dissolve in each other.

The second part of the answer has to do with a force called the buoyancy force. This is the force that causes some objects to float in water. Suppose you want to dissolve 1 cubic cm of oil in water. For this to happen, the oil has to displace 1 cubic cm of water. The buoyancy force on the oil is equal to the weight of that 1 cubic cm of water. Oil is less dense than water, so 1 cubic cm of oil weighs less than 1 cubic cm of water. Therefore, the upward buoyancy force on the oil, which is equal to the weight of water displaced, is greater than the downward force of gravity on the oil, also known as the weight of the oil. This inequality of forces causes the oil to rise in the water. If the oil were denser than the water, the oil's weight (the downward force) would exceed the buoyancy force (the upward force), and the oil would sink in the water.
Answered by: Philip Zell, Ph.D. Physics, ACT, Inc.