Intermolecular forces are the attractions between molecules, and they control nearly every physical property you can observe: whether a substance is solid, liquid, or gas at room temperature, how quickly it evaporates, whether it dissolves in water, and how it behaves on a surface. These forces are much weaker than the bonds holding atoms together within a molecule, typically ranging from about 5 to 50 kJ/mol compared to hundreds of kJ/mol for a covalent bond. But their collective effects shape the physical world around you and drive critical biological processes like DNA stability and protein folding.
The Four Types and Their Relative Strength
Not all intermolecular forces are equal. The weakest are London dispersion forces, with an energy of roughly 5 kJ/mol. Every molecule experiences these, even nonpolar ones. They arise because electrons are constantly moving, creating fleeting imbalances in charge that induce attractions between neighboring molecules. Larger molecules with more electrons have stronger dispersion forces because their electron clouds are easier to distort, a property called polarizability. This is why heavier molecules tend to have higher boiling points even when they lack any permanent charge separation.
Dipole-dipole interactions are stronger, around 15 kJ/mol. These occur between polar molecules, where the slightly positive end of one molecule is attracted to the slightly negative end of another. Hydrogen bonding is a particularly strong version of this: when hydrogen is bonded to oxygen, nitrogen, or fluorine, the resulting attraction to a neighboring molecule can range from about 4 to 63 kJ/mol depending on the system. Water’s unusual properties, from its high boiling point to its remarkable surface tension, come from extensive hydrogen bonding between its molecules.
The strongest intermolecular force is the ion-dipole interaction, at roughly 50 kJ/mol. This is what happens when an ion (like sodium from dissolved table salt) interacts with polar molecules like water. It’s the driving force behind why salts dissolve so readily in water.
They Determine Boiling and Melting Points
The most direct thing intermolecular forces do is set the temperature at which a substance changes phase. To boil a liquid, you need to give its molecules enough kinetic energy to break free from their neighbors. Stronger intermolecular forces mean you need more energy, which means a higher boiling point. This is why water, with its strong hydrogen bonds, boils at 100°C, while a similarly sized molecule like methane, held together only by weak dispersion forces, boils at negative 161°C.
Melting works on the same principle but involves a different transition. In a solid crystal, molecules are locked into a regular, tightly packed arrangement. Intermolecular forces constrain their motion far more than in a liquid. The melting point reflects how much thermal energy is needed to shake molecules loose from that ordered structure into the more random arrangement of a liquid. Substances with stronger attractions between molecules require more thermal energy to make that jump.
They Control Evaporation and Vapor Pressure
At any given temperature, some molecules at the surface of a liquid are moving fast enough to escape into the air. How many manage to escape depends on intermolecular force strength. Substances with weak intermolecular forces let molecules escape easily, producing a high vapor pressure. These are the volatile substances: rubbing alcohol, gasoline, nail polish remover. You can smell them immediately because their molecules readily enter the gas phase.
Substances with strong intermolecular forces hold their molecules more tightly, so fewer escape at a given temperature. They have low vapor pressure and evaporate slowly. This is why a puddle of water takes much longer to dry than a puddle of rubbing alcohol, even though both are liquids at room temperature. Water’s hydrogen bonds keep its molecules bound to one another more effectively.
They Shape Surface Tension and Viscosity
Surface tension is the tendency of a liquid’s surface to resist being stretched or broken. It happens because molecules at the surface are pulled inward and sideways by their neighbors but have no molecules above them pulling outward. The stronger the intermolecular forces, the harder the surface resists disruption. Water has an unusually high surface tension for its size, which is why small insects can walk on it and why water forms rounded droplets on a waxed car hood. As water heats up, its molecules move faster and overcome more of those hydrogen bonds, so surface tension drops.
Viscosity, or resistance to flow, follows the same logic. Molecules in a viscous liquid have a harder time sliding past each other because of strong intermolecular attractions between them. Structurally complex molecules with many points of contact also tend to be more viscous. Honey is thick partly because its sugar molecules form extensive hydrogen bonds with one another and with water. Motor oil is viscous because its long hydrocarbon chains generate strong cumulative dispersion forces.
They Decide What Dissolves in What
The old chemistry rule “like dissolves like” is really a statement about intermolecular forces. For one substance to dissolve in another, the new attractions formed between solute and solvent molecules need to be roughly as strong as the ones being broken. When the intermolecular forces are mismatched, dissolving doesn’t happen.
This is why sugar dissolves easily in water but not in gasoline. Glucose has five groups capable of hydrogen bonding with water, making it highly soluble (about 91 grams in 120 mL of water). A nonpolar substance like elemental sulfur, held together only by dispersion forces, won’t dissolve in water at all because breaking water’s hydrogen bonds costs far more energy than the weak new attractions could compensate for. Sulfur dissolves just fine in nonpolar solvents where the intermolecular forces on both sides are comparable.
Nonpolar gases like oxygen and nitrogen follow the same pattern. They can only interact with solvents through dispersion forces, so they dissolve much better in nonpolar solvents than in water. This force-matching principle governs everything from how your body absorbs nutrients to how industrial solvents are chosen.
They Hold Biological Structures Together
Some of the most important work intermolecular forces do happens inside your cells. The double helix shape of DNA is maintained by hydrogen bonds between paired bases on opposite strands. These bonds are strong enough to keep the two strands together under normal conditions but weak enough to be pulled apart when a cell needs to copy its genetic information. That balance is essential for life.
Proteins rely on the same forces to function. About two-thirds of the hydrogen bonds within a typical protein are between backbone segments, and these are what stabilize the coiled and folded shapes (alpha helices and beta sheets) that make up a protein’s core structure. Without these intermolecular attractions, proteins would be floppy, shapeless chains incapable of doing their jobs as enzymes, structural supports, or molecular signals. The specificity of how proteins bind to DNA also depends on a precise balance of hydrogen bond strengths at the interface between the two molecules.
Dispersion forces play a supporting role in biology too. The interiors of folded proteins are largely nonpolar, and the cumulative effect of many weak dispersion forces helps keep those hydrophobic cores packed tightly together. Cell membranes hold their structure partly through dispersion forces between the long hydrocarbon tails of their lipid molecules.