Passage VII

Vapor pressure is the pressure exerted by the vapor above a liquid in a closed container at a given temperature and at equilibrium. At equilibrium, the rate of evaporation equals the rate of condensation.

Two students observed that the vapor pressure of H₂O decreased when NaCl was dissolved in the H₂O. The students then offered explanations for why this decrease occurred.

Student 1

Solvent molecules are attracted to charged solute particles. This causes solvent molecules to surround solute particles, forming clusters. Additional energy is needed to break solvent molecules free from these clusters. Thus, at a given temperature, the vapor pressure of the solution is lower than that of the pure liquid. The vapor pressure decreases as the concentration of charged solute particles increases and as the magnitude of the charge on each solute particle increases. Neutral solute particles do not affect the vapor pressure of a liquid.

Student 2

Vapor pressure decreases because some of the solvent particles that have sufficient energy to escape the solution's surface collide with the solute particles and lose energy. Thus, at a given temperature, the vapor pressure of the solution is lower than that of the pure liquid. The size of the decrease in vapor pressure is directly proportional to the concentration of solute particles.A neutral solute particle decreases the vapor pressure of a liquid by the same amount as does a charged solute particle.The concentration of solute particles is the only factor that affects the vapor pressure of a liquid.

Experiment

Three 1 kg solutions were made by dissolving 1.0 mole (6 × 10²³ units or molecules) of glucose, NaCl, or CaCl₂, respectively, in distilled H₂O. Thus, the concentration of each solution was 1.0 mole/kg. Table 1 shows the solute particle(s) resulting from each unit (or molecule) of each substance dissolved in H₂O. The students then measured the vapor pressure of distilled H₂O and of each solution at 25℃.