Why do oil and water not mix?

When attempting to understand why oil and water do not mix, we must start by examining their chemical structures. Water is composed of two hydrogen atoms bonded to one oxygen atom in a V-shape. This arrangement results in a polar molecule, which has a partial positive charge near the hydrogen atoms and a partial negative charge near the oxygen atom.

In contrast, oil is predominantly made up of long chains of hydrocarbons. These chains consist of carbon and hydrogen atoms linked together without a significant difference in electronegativity, resulting in nonpolar molecules. The lack of polarity means that oil molecules do not have charged ends that can interact with the charged ends of water molecules. The disparity between the polar structure of water and the nonpolar nature of oil leads them to be immiscible; that is, they do not mix.

Hydrogen bonding plays a pivotal role in the interaction between molecules, particularly in a substance like water. Water's polarity allows it to form hydrogen bonds, which are strong attractions between the partial negative charge of the oxygen atom of one water molecule and the partial positive charge of the hydrogen atom of another water molecule. This creates a network of interconnected water molecules.

These bonds are a significant factor in the unique properties of water, such as its high boiling point and the fact that it expands when it freezes. Hydrogen bonds require a considerable amount of energy to break, explaining why nonpolar molecules, such as those in oil, cannot easily insert themselves between water molecules. As a result, instead of dissolving and mixing with water, oil remains separate, forming clusters of oil molecules.

Density differences between oil and water further explain their lack of mixing. Oil is less dense than water, a characteristic that causes oil to float on water. This is a clear visual cue of immiscibility—the inability of two substances to combine into a homogeneous mixture.

When oil is added to water, the denser water molecules pack closely due to their intermolecular attraction and hydrogen bonding. Oil molecules, being nonpolar, are excluded from this tight packing and remain on top due to their lighter density. This separation is reinforced by the fact that water molecules preferentially form hydrogen bonds with each other over interacting with nonpolar oil molecules, leading to a clear partition between the two liquids.