A nonvolatile substance is a material that resists the tendency to evaporate easily into a gaseous state under standard environmental conditions, such as typical room temperature and atmospheric pressure. Nonvolatile compounds remain stable in their solid or liquid forms, possessing a slow rate of evaporation. This characteristic is fundamental in chemistry and physics, and understanding this property is necessary for countless applications, from food preservation to industrial product formulation.
Defining Nonvolatility and the Role of Vapor Pressure
The defining scientific mechanism behind a substance’s nonvolatility is its vapor pressure, which is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases in a closed system. Nonvolatile substances are characterized by an extremely low vapor pressure, meaning only a very small number of molecules possess enough energy to escape the liquid or solid surface and enter the gas phase. This low tendency to vaporize is a direct consequence of the strong attractive forces holding the molecules together in the condensed state.
These powerful intermolecular forces often include ionic bonds, extensive networks of strong hydrogen bonds, or very large van der Waals forces common in high-molecular-weight compounds. Breaking these strong attractions requires a significant input of thermal energy, which translates to a high boiling point. For example, the strong ionic attraction between sodium and chloride ions in table salt prevents it from easily transitioning into a vapor.
Contrast with Volatile Substances
Substances are classified as volatile when they possess a high vapor pressure and a low boiling point, allowing them to evaporate quickly even at room temperature. This rapid rate of evaporation is due to the comparatively weak intermolecular forces between their molecules. These weaker forces require only a small amount of thermal energy to be overcome, enabling molecules to escape the liquid surface.
Volatile substances are often composed of small, nonpolar molecules, which primarily rely on London dispersion forces for attraction. Examples like rubbing alcohol, gasoline, or perfume are noticeable because their molecules quickly transition into the air, often resulting in a distinct odor. The difference between alcohol and table salt illustrates this contrast: alcohol evaporates rapidly from an open container, while salt remains a stable solid.
A volatile substance’s molecules contribute significantly to the total pressure above the liquid, whereas a nonvolatile substance contributes negligibly. This fundamental difference in vapor pressure is the physical basis for their contrasting behavior at standard temperatures.
Common Examples and Practical Applications
Nonvolatile substances are integral to daily life and industry, encompassing a wide range of materials from simple salts to complex polymers. Common examples include mineral salts like sodium chloride, large organic molecules such as glucose and sucrose, and high-molecular-weight proteins. Glycerin, a nonvolatile liquid, is also frequently used in pharmaceuticals and cosmetics.
The nonvolatile nature of these compounds is often exploited in applications where stability and lack of evaporation are necessary. For instance, in material science, nonvolatile compounds are used as corrosion inhibitors or as non-evaporating components in specialty oils. Their stability at high temperatures makes them suitable for industrial processes like high-temperature material fabrication.
When a nonvolatile substance is dissolved into a volatile liquid, such as salt in water, it affects the physical properties of the solvent in predictable ways. These changes are known as colligative properties, which depend only on the number of solute particles present, not their chemical identity. The presence of the nonvolatile solute effectively lowers the vapor pressure of the solvent.
This vapor pressure lowering leads directly to boiling point elevation, where the solution requires a higher temperature to boil than the pure solvent. Conversely, it also causes freezing point depression, lowering the temperature at which the solution will freeze. These effects are utilized in applications like antifreeze solutions, which use nonvolatile solutes to manage engine coolant temperatures.