Vegetable oil does not dissolve in water, which results in two distinct layers when combined. Solubility describes the ability of a solute to mix completely and uniformly with a solvent to form a single phase solution. Water, often called the “universal solvent,” fails to dissolve vegetable oil because of the fundamental differences in their molecular properties and how their individual molecules interact.
The Rule of “Like Dissolves Like”
The primary principle that governs whether two substances will mix is often summarized by the phrase “like dissolves like.” This means that substances with similar molecular characteristics will readily dissolve in one another, while those with fundamentally different characteristics will not.
At the molecular level, this compatibility is determined by the distribution of electrical charge within the molecules, a property known as polarity. Polar compounds have an uneven charge distribution, resulting in partial positive and negative ends, allowing them to attract other polar molecules. Nonpolar compounds have an even distribution of charge and lack these distinct ends. Polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes, but a polar substance will not dissolve a nonpolar one.
Polarity: Contrasting Water and Vegetable Oil
Water is a highly polar molecule due to the arrangement of its atoms. The oxygen atom exerts a stronger pull on the shared electrons, creating a partial negative charge near the oxygen and partial positive charges near the hydrogen atoms. This uneven sharing causes water molecules to strongly attract one another through hydrogen bonds.
Vegetable oil, composed primarily of triglycerides, is categorized as a nonpolar substance. Triglyceride molecules consist of three long fatty acid chains attached to a glycerol backbone. These chains are made up of carbon and hydrogen atoms that share electrons almost equally, resulting in a symmetrical, even distribution of electrical charge. Because oil molecules lack significant charge, they cannot form the strong attractive forces necessary to integrate with the highly-attracted water molecules.
The Physical Result of Mixing: Immiscibility
The physical consequence of this polarity mismatch is called immiscibility, meaning the two liquids cannot form a homogeneous solution. When oil is added to water, the water molecules preferentially bond with each other, effectively excluding the oil molecules. The oil molecules are left to associate with each other through much weaker intermolecular forces.
This separation is readily visible and is enforced by the difference in density between the two liquids. Vegetable oils have a density of approximately 0.8 g/cm³, which is less than the 1.0 g/cm³ density of water. Consequently, the less dense oil always floats, forming a layer on top of the water. If the mixture is vigorously shaken, the oil can be temporarily dispersed into tiny droplets, creating an unstable emulsion that quickly separates.