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When sugar dissolves in water, it remains in the liquid phase even as the water evaporates. This behavior exemplifies a nonvolatile solute—a substance that does not contribute appreciably to the vapor pressure of its solution.
A nonvolatile solute does not generate vapor pressure, meaning it cannot escape a solution as a gas.
A typical solution consists of a solvent and one or more solutes. Water is the most common solvent, and many solutes exhibit distinct behaviors when dissolved. Nonvolatile solutes, such as sucrose, sodium chloride, or proteins, have low vapor pressures and high boiling points, so they stay in the liquid phase even under elevated temperatures.
Volatility reflects a solute’s propensity to vaporize. Substances with boiling points below 100 °C (212 °F) are generally considered volatile; those above are nonvolatile. When a solution containing a volatile solute is heated, both the solvent and the solute can evaporate, producing a vapor that carries molecules of both components. In contrast, heating a solution of a nonvolatile solute results in vapor composed almost exclusively of the solvent, with the solute remaining dissolved.
Nonvolatile impurities raise a solution’s boiling point. Adding a nonvolatile solute to water reduces the number of free water molecules that can escape, lowering the partial vapor pressure of water and requiring higher temperatures to reach boiling. Volatile impurities, if they do not react with the solution, typically lower the boiling point because they increase the total vapor pressure. However, if a volatile impurity reacts chemically, the effect on boiling temperature becomes unpredictable.
