Water is the denser substance, which is why oil always floats on its surface, whether it is vegetable oil in a salad dressing or crude oil from a massive spill. This separation is rooted not in the total weight of the substances, but in density. Understanding this difference requires exploring how the molecules of each liquid are arranged.
Defining Density: The True Meaning of “Heavier”
The question of which liquid is “heavier” actually asks which one is denser. Density is a measure of how much mass is packed into a specific volume. It is calculated by dividing an object’s mass by its volume. When two liquids of equal volume are compared, the one with the greater density will have the greater mass.
Density determines whether a substance will float or sink when placed in another liquid. For example, a truckload of cotton may weigh more than a single brick, but the brick is still the denser material. Water is used as a benchmark for density, reaching approximately 1.0 gram per milliliter (g/mL). Any substance with a density less than 1.0 g/mL will float on water.
Water’s Molecular Structure and Polarity
Water’s relatively high density is a direct result of its molecular structure. A single water molecule consists of one oxygen atom bonded to two hydrogen atoms, forming a bent shape. Because the oxygen atom attracts electrons more strongly than the hydrogen atoms, the molecule is polar, meaning it has a slightly negative end near the oxygen and slightly positive ends near the hydrogen atoms.
This polarity allows water molecules to form strong connections called hydrogen bonds with neighboring water molecules. These bonds constantly form and break in liquid water, keeping the molecules tightly packed together. This close-knit arrangement allows a large amount of mass to occupy a small volume, resulting in water’s high density.
The ability to form these powerful, temporary bonds is what makes water denser than most other common liquids. Oil molecules, by contrast, lack this polarity, which prevents them from forming strong intermolecular attractions. This means oil molecules cannot pack together as closely as water molecules can.
How Oil Types Affect Density
The term “oil” encompasses various substances, such as vegetable oils and crude oil, but all are primarily composed of non-polar hydrocarbon chains. These long chains of carbon and hydrogen atoms are much larger than a water molecule. Although these larger molecules have greater individual mass, they cannot be arranged as compactly as water molecules.
This loose packing means that oils occupy more space for the same amount of mass compared to water, resulting in a lower overall density. For example, cooking oils like olive or soybean oil have densities around 0.91 to 0.93 g/mL, confirming they are less dense than water. This density difference is why oil and water are immiscible. Water molecules are more strongly attracted to each other than they are to the non-polar oil molecules. Consequently, the denser water settles underneath the oil, forcing the less dense oil to the surface and creating the distinct layers we observe.