Nonpolar substances dissolve in other nonpolar substances because their molecules interact through the same type of weak, temporary electrical attractions. When two nonpolar materials mix, the energy cost of separating their molecules is roughly equal to the energy gained from forming new interactions, so mixing happens easily. This principle is often summarized as “like dissolves like.”
How Nonpolar Molecules Attract Each Other
Nonpolar molecules have no permanent positive or negative ends. That might make it seem like they shouldn’t attract each other at all, but they do, through forces called London dispersion forces. These are the only type of intermolecular attraction that operates between nonpolar molecules, and understanding them is the key to understanding why nonpolar dissolves nonpolar.
Electrons are constantly moving around every atom. At any given instant, the electrons in a molecule are not perfectly evenly distributed. One side of the molecule briefly has slightly more electron density, giving it a fleeting negative charge, while the other side becomes slightly positive. This creates a temporary, or “instantaneous,” dipole that lasts only a fraction of a second before the electrons shift again.
When two nonpolar molecules are close together, the temporary dipole on one molecule influences the electrons in its neighbor. The nearby molecule’s electrons get nudged away from the negative end and toward the positive end, creating a matching temporary dipole. For that instant, the two molecules attract each other, positive end to negative end. These synchronized fluctuations happen continuously across billions of molecules, producing a net attractive force that holds nonpolar liquids and solids together. Carbon tetrachloride, for example, exists as a liquid at room temperature entirely because of these flickering attractions between its molecules.
Why the Energy Balance Works Out
Dissolving anything requires three energy steps. First, molecules of the solute (the substance being dissolved) must be pulled apart from each other. Second, molecules of the solvent must be pushed aside to make room. Third, the solute and solvent molecules form new attractions with each other. For dissolving to happen spontaneously, the energy released in step three needs to roughly compensate for the energy spent in steps one and two.
When a nonpolar solute meets a nonpolar solvent, all three steps involve the same type of force: London dispersion forces. The strength of attraction between two nonpolar solute molecules is similar to the strength between two solvent molecules and similar again to the strength between a solute molecule and a solvent molecule. The energy books essentially balance. There’s no large penalty to pay and no large gain, so the molecules mix freely.
There’s also an entropy factor working in favor of mixing. When two substances combine, the molecules have more possible arrangements than when they’re separated. This increase in disorder makes the mixed state more thermodynamically favorable, giving an extra push toward dissolving even when the energy balance is nearly neutral.
Why Nonpolar Substances Won’t Dissolve in Water
The flip side of “like dissolves like” is that unlike substances resist mixing. Water molecules are polar, with strong permanent charges that form a tight network of hydrogen bonds. Each water molecule can form up to four hydrogen bonds with its neighbors, creating a highly organized structure. The energy holding this network together is substantial: breaking a single hydrogen bond costs about 11.35 kJ/mol, far more than the weak London dispersion forces a nonpolar molecule could offer in return.
When you try to dissolve a nonpolar molecule in water, water’s hydrogen bond network has to break open to make room. The nonpolar molecule can only offer back weak London dispersion forces, which don’t come close to compensating for the lost hydrogen bonds. Around small nonpolar molecules, water can rearrange itself into cage-like structures without losing too many hydrogen bonds, but this reorganization restricts the water molecules’ freedom of movement, causing a large drop in entropy. Around larger nonpolar molecules, hydrogen bonds at the surface are broken outright, creating an energy penalty on top of the entropy loss.
This is why oil and water don’t mix. It’s not that oil molecules are repelled by water. It’s that water molecules are so strongly attracted to each other that incorporating a nonpolar intruder is energetically unfavorable. The water essentially squeezes the nonpolar molecules out, forcing them together in what’s known as the hydrophobic effect.
Matching Solubility in Practice
Chemists can predict whether two substances will dissolve in each other using a value called the Hildebrand solubility parameter, which captures how strongly a substance’s molecules attract one another. The rule is straightforward: if two materials have solubility parameters within about 2 MPa1/2 of each other, they’ll generally mix well. If the difference is larger than that, they won’t. This approach works with about 70 to 75% accuracy for nonpolar materials, which makes sense given that nonpolar substances all rely on the same type of intermolecular force and tend to cluster in a similar range of parameter values.
This is why you can predict solubility just by looking at molecular structure. Hexane (a nonpolar hydrocarbon) dissolves in toluene (another nonpolar hydrocarbon) because their molecules interact with similar strength. But hexane won’t dissolve in water because the two substances sit at completely different points on the solubility scale.
Everyday and Industrial Examples
You encounter nonpolar-dissolves-nonpolar chemistry more often than you might think. Grease on your hands won’t wash off with water alone because grease is nonpolar and water is polar. But it dissolves easily in nonpolar solvents. This is exactly why the U.S. Environmental Protection Agency uses n-hexane, a nonpolar solvent, as its standard method for extracting oils, greases, waxes, animal fats, and hydrocarbons from water samples. Hexane pulls these nonpolar materials out of water because they’d rather be surrounded by hexane molecules than trapped in water’s hydrogen bond network.
Nail polish remover works on the same principle. The nonpolar components of nail polish dissolve in organic solvents that share similar intermolecular forces. Paint thinner dissolves oil-based paint for the same reason. Dry cleaning uses nonpolar or low-polarity solvents to lift oily stains from fabric without water, which would fail to dissolve them.
In your body, this chemistry governs how you absorb fat-soluble vitamins (A, D, E, and K). These vitamins are nonpolar, so they dissolve in the dietary fats you eat rather than in the watery environment of your digestive tract. Your body packages them into fat-based transport particles to shuttle them through your bloodstream, which is water-based. Without dietary fat to dissolve them first, absorption of these vitamins drops significantly.
Molecular Size Affects Strength
Not all nonpolar substances interact with equal strength. London dispersion forces get stronger as molecules get larger, because bigger molecules have more electrons and larger electron clouds that are easier to distort into temporary dipoles. This is why methane (a small nonpolar molecule with just one carbon) is a gas at room temperature, while octane (a chain of eight carbons) is a liquid, and polyethylene (thousands of carbons) is a solid. They’re all nonpolar, but the cumulative strength of London dispersion forces increases with molecular size.
This also means that two nonpolar substances of very different molecular sizes may not dissolve in each other as readily as two that are more similar. A very large nonpolar polymer, for instance, may swell but not fully dissolve in a small nonpolar solvent, because the energy balance shifts when the molecules are mismatched in size. The general “like dissolves like” rule still applies, but “like” refers to more than just polarity. Similar molecular size and shape improve solubility too.