When 1 mL of toluene is added to 100 mL of benzene (bp 80.1 °C), the boiling point of the benzene solution rises, but when 1 mL of benzene is added to 100 mL of toluene (bp 110.6 °C), the boiling point of the toluene solution falls. Explain.

Boiling point elevation is a colligative property that describes how the boiling point of a solvent increases when a non-volatile solute is added. This phenomenon occurs because the presence of solute particles disrupts the solvent's ability to evaporate, requiring a higher temperature to reach the boiling point. In the case of toluene and benzene, adding toluene to benzene raises the boiling point due to this effect.

Boiling point depression is another colligative property, which occurs when the boiling point of a solvent decreases upon the addition of a solute. This can happen if the solute has a lower boiling point than the solvent, leading to a reduction in the overall vapor pressure of the solution. When benzene is added to toluene, the boiling point of the resulting solution decreases because benzene's lower boiling point influences the overall vapor pressure.

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase. The intermolecular forces between molecules in a solution affect the vapor pressure; stronger interactions lead to lower vapor pressures. In the case of toluene and benzene, the differences in their intermolecular forces can explain why the addition of one to the other results in different boiling point behaviors, as the solute can either enhance or diminish the vapor pressure of the solvent.