nonvolatile

Meaning: A nonvolatile substance has negligible vapor pressure at the temperature of interest, because strong attractions (ionic bonds, extensive hydrogen bonding, large size) keep its particles in the liquid or solid. Salts and sugars are typical. A nonvolatile solute adds almost nothing to the vapor and dilutes the solvent at the surface, so fewer solvent molecules escape and vapor pressure falls. This underlies boiling point elevation and freezing point depression.

Equation: Psolution = Xsolvent × P°solvent, where Psolution is the solution's vapor pressure (Pa), Xsolvent is the solvent mole fraction (unitless), and P°solvent is the pure solvent's vapor pressure at the same temperature (Pa).

On the MCAT: Passages compare solutions such as NaCl, glucose and CaCl₂ and ask which has the lowest vapor pressure or highest boiling point. Rank by total dissolved particles (van't Hoff factor × molality, i × m), not by solute identity. On a vapor pressure versus temperature graph, the solution's curve lies below the pure solvent's.

Common confusion: Nonvolatile does not mean insoluble or unreactive; it means little evaporation. The simple Raoult's law form above applies when only the solvent is volatile; if both components are volatile, each adds to the total vapor pressure.