Vapor pressure (VP) describes the pressure exerted by a substance’s gas phase when it is in equilibrium with its liquid phase within a closed system. This pressure originates from molecules escaping the liquid surface. When a non-volatile substance, such as table sugar (sucrose), is dissolved in a solvent, the resulting solution exhibits vapor pressure lowering.
This occurs because the sugar molecules do not evaporate but occupy space at the liquid’s surface. The presence of these solute particles physically hinders the solvent molecules from transitioning into the vapor phase, resulting in a measurable reduction in the pressure exerted by the solvent vapor. The vapor pressure of a sugar solution is influenced by three main factors: the nature of the solvent, the amount of sugar dissolved, and the system’s temperature.
The Baseline: Nature of the Solvent
The most significant determinant of a solution’s vapor pressure is the initial, pure vapor pressure of the solvent itself, which establishes the absolute upper limit for the solution’s VP. This baseline is linked to the strength of the intermolecular forces (IMFs) holding the solvent molecules together. Liquids with strong IMFs require more energy for their molecules to break free and enter the gas phase.
Water, the common solvent for sugar, possesses strong hydrogen bonds that tightly link individual water molecules. These strong attractions mean that at any given temperature, fewer water molecules have sufficient kinetic energy to overcome the forces and escape the liquid surface, leading to a relatively low inherent vapor pressure. If sugar were dissolved in a solvent with weaker forces, such as ethanol, the baseline vapor pressure would be substantially higher. The nature of the solvent dictates the maximum vapor pressure the final sugar solution can achieve.
The Primary Influence: Solute Concentration
The most direct factor affecting the vapor pressure is the concentration of the dissolved sugar, specifically the number of solute particles relative to the solvent. This relationship is characteristic of colligative properties, which depend solely on the ratio of particles, not the chemical identity of the solute. The sugar molecules at the liquid-air interface obstruct the escape of solvent molecules, reducing the rate of vaporization.
The extent of this vapor pressure lowering is quantified by the concept of mole fraction, which is the ratio of the moles of solvent to the total moles of both solvent and solute. Raoult’s Law states that the solution’s vapor pressure is directly proportional to the solvent’s mole fraction. As more sugar is added, the solvent’s mole fraction decreases, and the resulting vapor pressure is lowered proportionally.
For instance, if the solvent’s mole fraction is \(0.95\), the solution’s vapor pressure will be \(95\%\) of the pure solvent’s vapor pressure at the same temperature. Sugar (sucrose, \(\text{C}_{12}\text{H}_{22}\text{O}_{11}\)) dissolves but does not dissociate into multiple ions. This means one molecule of sugar contributes one particle to the total count, making the concentration-dependent lowering straightforward to predict.
The Role of Temperature
Temperature is an external factor that governs the absolute magnitude of the vapor pressure for any liquid or solution. It directly influences the average kinetic energy of the molecules within the liquid. As the temperature of the solution increases, the solvent molecules gain more energy and move faster.
This greater kinetic energy allows a larger fraction of the solvent molecules to overcome the attractive forces holding them in the liquid phase. The result is an increased rate of molecules transitioning into the vapor phase, which leads to a higher vapor pressure. This dependency is not linear; the vapor pressure increases exponentially with rising temperature.
While concentration determines the degree of vapor pressure lowering relative to the pure solvent, temperature dictates the total pressure value. A sugar solution at a higher temperature will always have a higher vapor pressure than the same solution at a lower temperature. The two factors work in tandem: concentration sets the proportional reduction, and temperature sets the energy baseline.