In chemistry, nonvolatile describes a substance that does not easily evaporate into a gas at room temperature and normal pressure. A nonvolatile substance has very low vapor pressure, meaning its molecules stay in liquid or solid form rather than escaping into the air. Table salt, sugar, and vegetable oil are common examples. The opposite, a volatile substance, readily evaporates: think rubbing alcohol, gasoline, or nail polish remover.
Why Some Substances Evaporate and Others Don’t
Whether a substance is volatile or nonvolatile comes down to how strongly its molecules hold onto each other. These attractive forces between molecules, called intermolecular forces, act like invisible glue. In any liquid, molecules are constantly moving at different speeds. A few will always have enough energy to break free from the surface and enter the gas phase. That’s evaporation.
In a nonvolatile substance, the intermolecular forces are strong enough that very few molecules can escape at room temperature. It takes significantly more energy to pull them apart. This is why nonvolatile liquids tend to have high boiling points: you need to add a lot of heat before the molecules move fast enough to overcome those forces and become a gas. Honey, motor oil, and molten metals are all nonvolatile for this reason.
Volatile substances, by contrast, have weaker intermolecular forces. Their molecules break free easily, which is why you can smell gasoline from across a parking lot. The EPA classifies volatile organic compounds (VOCs) partly by boiling point: the lower the boiling point, the more volatile the compound. Very volatile compounds exist almost entirely as gases in the air, while the least volatile compounds remain mostly in solids, liquids, or on surfaces like dust and building materials.
How Nonvolatile Solutes Change a Solution
The concept of nonvolatility becomes especially important when you dissolve a nonvolatile substance in a liquid. Adding table salt or sugar to water changes the water’s physical properties in predictable ways. These changes, called colligative properties, depend only on how many solute particles are dissolved, not on what those particles are.
Vapor Pressure Drops
When you dissolve a nonvolatile solute in water, fewer water molecules sit at the surface where they can escape into the gas phase. The result is lower vapor pressure above the solution compared to pure water. This relationship is captured by Raoult’s Law, which states that the vapor pressure of the solvent above a solution is directly proportional to the fraction of solvent molecules present. A nonvolatile solute contributes zero vapor pressure of its own, so the total vapor pressure simply decreases as you add more solute.
Boiling Point Rises
Because the solution has lower vapor pressure than pure water, it needs to be heated to a higher temperature before it boils. Boiling happens when vapor pressure equals the external air pressure, so a solution with reduced vapor pressure has to get hotter to reach that threshold. This is why salted water for pasta takes slightly longer to reach a rolling boil. The more solute you add, the higher the boiling point climbs.
Freezing Point Drops
Nonvolatile solutes also lower the freezing point of a solution. The change in freezing point is directly proportional to the concentration of dissolved particles. This is the principle behind road salt in winter: dissolving salt in the thin layer of water on icy roads pushes the freezing point below the ambient temperature, turning ice back into liquid. The identity of the solute doesn’t matter for this effect. Sugar would work too, though salt is cheaper and dissociates into more particles per unit, giving it a bigger impact per gram.
Volatile vs. Nonvolatile in Everyday Life
The distinction between volatile and nonvolatile shows up in places you might not expect. Perfume, for instance, is a mix of both: volatile compounds give you the initial burst of scent, while less volatile ones linger for hours. Paint releases volatile organic compounds as it dries, which is why fresh paint has a strong smell that fades over time. Once the volatile solvents evaporate, the nonvolatile resins and pigments remain as a solid film on your wall.
VOCs are a genuine health and environmental concern. Indoors, they can irritate airways and cause other health effects in sensitive people. Outdoors, the EPA regulates them primarily because they react with nitrogen oxides and sunlight to form ground-level ozone, a key ingredient in smog. Nonvolatile compounds pose far less inhalation risk simply because they don’t become airborne easily.
How Nonvolatile Substances Are Stored
In a laboratory, volatile and nonvolatile chemicals are stored very differently. Volatile poisons require flammable cabinets or explosion-proof refrigerators because the primary danger is inhaling their fumes. Nonvolatile liquid poisons, on the other hand, are stored in standard cabinets or refrigerators. The main concern is preventing skin contact or accidental reactions with other chemicals, not airborne exposure. Larger containers of nonvolatile liquid poisons are typically kept below bench level on low shelves to reduce the risk of spills. This practical difference in handling reflects the core distinction: nonvolatile substances stay put, while volatile ones are constantly trying to escape into the air around them.