The boiling point of a liquid is defined as the temperature at which its vapor pressure is equal to the external pressure. Explain how the boiling point of a solution of the same liquid containing a nonvolatile solute is higher, and the vapor pressure is lower, than that of the pure substance.
When a nonvolatile solute is added to a liquid, it reduces the vapor pressure of the solution compared to the pure liquid. This is because the solute molecules occupy space at the surface of the liquid, making it harder for the solvent molecules to escape and vaporize. As a result, a higher temperature is required for the vapor pressure of the solution to equal the external pressure, which is the definition of the boiling point.
In other words, the presence of a nonvolatile solute interferes with the vaporization of the solvent molecules, leading to a higher boiling point for the solution compared to the pure solvent. This phenomenon is known as boiling-point elevation.
In addition, the boiling point of a solution is also higher than that of the pure substance because the solute molecules increase the entropy of the solution, making it less energetically favorable for the solvent molecules to escape and vaporize. This further contributes to the lowering of the vapor pressure of the solution.
Overall, the boiling point of a solution containing a nonvolatile solute is higher and the vapor pressure is lower than that of the pure substance due to the interference of the solute molecules with the vaporization process.