Compared to the pure solvent, the boiling point of a solution is
A. Increased because the solution has a higher osmotic pressure.
B. Decreased because the solute boils at a higher temperature than the solvent.
C. Decreased because the dissolved solute raises the vapor pressure of the solution over that of the solvent.
D. Increased because of the lowered vapor pressure of the solution.
Answers
Answer:
Boiling Point Elevation and Freezing Point Depression
The figure below shows the consequences of the fact that solutes lower the vapor pressure of a solvent. The solid line connecting points B and C in this phase diagram contains the combinations of temperature and pressure at which the pure solvent and its vapor are in equilibrium. Each point on this line therefore describes the vapor pressure of the pure solvent at that temperature. The dotted line in this figure describes the properties of a solution obtained by dissolving a solute in the solvent. At any given temperature, the vapor pressure of the solvent escaping from the solution is smaller than the vapor pressure of the pure solvent. The dotted line therefore lies below the solid line.
phase diagram
The decrease in the vapor pressure of the solvent that occurs when a solute is added to the solvent causes an increase in the boiling point and decrease in the melting point of the solution.
According to this figure, the solution can't boil at the same temperature as the pure solvent. If the vapor pressure of the solvent escaping from the solution is smaller than the vapor pressure of the pure solvent at any given temperature, the solution must be heated to a higher temperature before it boils. The lowering of the vapor pressure of the solvent that occurs when it is used to form a solution therefore increases the boiling point of the liquid.