By Deborah Walden • Updated March 24, 2022
Oil spills, salad dressings, and countless everyday moments remind us of a simple yet fundamental scientific fact: oil and water do not mix. The behavior of these two liquids is dictated by the smallest building blocks that compose them—molecules—and their electrical properties.
Water (H₂O) is a polar molecule because the oxygen atom carries a partial negative charge while the hydrogen atoms carry partial positive charges. This uneven charge distribution creates a dipole. In contrast, most oils are composed of long hydrocarbon chains that lack any significant charge separation, making them nonpolar.
Opposite electrical charges attract, so the negative side of one water molecule is drawn to the positive side of another. This attraction forms hydrogen bonds—strong, directional links that give water its cohesion. When an oil molecule encounters water, the oil’s nonpolar nature means it has a weaker pull toward water than it does toward its own kind. However, the hydrogen bonds between water molecules are so robust that they resist being broken by oil.
Because the hydrogen bonds that bind water molecules together are stronger than the fleeting attractions between oil and water, the oil cannot penetrate the water network. If a drop of oil is placed gently on a surface of water, it spreads into a film one molecule thick, then retreats to form a distinct layer. Shake the mixture and the oil will quickly re‑form into separate globules once the motion stops.
Water’s density is higher than that of most oils, so water molecules are packed more tightly. When oil is introduced, the stronger water–water bonds prevent oil molecules from infiltrating the bulk of the liquid, causing the oil to be pushed upward. Consequently, oil always rises to the surface and remains separated from the underlying water.
In short, oil and water stay apart because water’s polarity and hydrogen‑bonding network dominate, while oil’s nonpolarity and lower density keep it buoyant on the surface.
