Nonpolar molecules do not dissolve in water due to fundamental differences in their molecular structures and the forces that hold them together. Water, often called the universal solvent, is highly effective at dissolving many substances, but it excludes molecules that lack a similar electrical nature. Understanding this requires examining molecular polarity, which dictates how any substance will interact when mixed with water.
Defining Molecular Polarity
Molecular polarity describes the distribution of electrical charge within a molecule. This distribution is determined by the difference in electronegativity—the ability of an atom to attract electrons—between the bonded atoms. In a polar molecule, like water (H2O), the oxygen atom pulls the shared electrons closer than the hydrogen atoms, creating unequal sharing. This results in a partial negative charge on the oxygen side and a partial positive charge on the hydrogen side, forming a molecular dipole. Molecules with this separation of charge are considered polar.
Nonpolar molecules, by contrast, have electrons shared equally (such as in molecules made of identical atoms), or they possess a symmetrical shape that causes internal dipoles to cancel out. Examples of nonpolar substances include fats, oils, and hydrocarbons like methane.
The General Rule of Solubility
The ability of one substance to dissolve in another is governed by the foundational principle in chemistry: “like dissolves like.” This rule asserts that a solvent will dissolve a solute if they share similar molecular polarity. Polar solvents, such as water, are excellent at dissolving other polar molecules and ionic compounds. Conversely, nonpolar solvents, like gasoline, readily dissolve nonpolar solutes, such as grease, wax, and oils.
Dissolution requires that the attractive forces between the solute and solvent molecules be strong enough to overcome the forces holding the individual molecules together. If the intermolecular forces of the two substances are incompatible, the solute will not dissolve.
Why Water Rejects Nonpolar Substances
The failure of nonpolar molecules to dissolve in water is primarily an energetic issue rooted in the strong attraction water molecules have for each other. Water molecules are held together by powerful intermolecular forces called hydrogen bonds, which are an intense form of dipole-dipole attraction. To dissolve a nonpolar substance, water must break these strong hydrogen bonds to make space for the new molecule.
When water molecules surround a nonpolar molecule, they form significantly weaker attractions, specifically London dispersion forces. This trade-off is energetically unfavorable because replacing strong water-water interactions with weak water-nonpolar interactions requires a net input of energy.
Water molecules respond by minimizing contact with the nonpolar substance, forcing the nonpolar molecules to aggregate. This phenomenon is known as the hydrophobic effect, or “water-fearing,” and is driven by water’s desire to maximize its strong hydrogen bonding network. By excluding the nonpolar substance, water molecules regain favorable water-water interactions. The nonpolar molecules are squeezed out of the water phase, leading to visible separation into distinct layers.
Common Examples in Everyday Life
The most common demonstration of this principle is the separation of oil and water. Salad dressings or a greasy pot of water show nonpolar oil molecules coalescing into a separate layer, often floating on the denser, polar water. Oil molecules, which are typically large hydrocarbons, stick to each other to minimize disruptive contact with the surrounding water.
Substances like soap or detergents are designed to overcome this natural aversion. These molecules are amphiphilic, meaning they possess both a polar, water-attracting end and a long, nonpolar, oil-attracting hydrocarbon tail. When soap is added to water and grease, the nonpolar tails burrow into the grease droplet, while the polar heads remain on the exterior, facing the water. This arrangement forms a tiny, water-soluble sphere called a micelle, trapping the grease inside.
The polar exterior of the micelle allows the structure to be carried away by the water, enabling the nonpolar grease to be washed down the drain. This mechanism bypasses the energetic barrier by creating a stable interface between the two incompatible substances.