The following phase diagram shows part of the vapor- pressure curves for a pure liquid (green curve) and a solution of the first liquid with a second volatile liquid (red curve).
(a) Is the boiling point of the second liquid higher or lower than that of the first liquid?

Verified step by step guidance

1. Identify the vapor-pressure curves for the pure liquid (solvent) and the solution from the phase diagram.
2. Understand that the boiling point is the temperature at which the vapor pressure of the liquid equals the external pressure (usually 1 atm).
3. Observe the temperatures at which the vapor pressure of the solvent and the solution reach 1 atm.
4. Note that the green curve (solution) reaches 1 atm at a higher temperature compared to the blue curve (solvent).
5. Conclude that the boiling point of the solution (second liquid) is higher than that of the pure solvent (first liquid).

Key Concepts

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It indicates how readily a substance evaporates; higher vapor pressure means a substance evaporates more easily. In the context of the phase diagram, the vapor pressure curves show how the pressure changes with temperature for both the pure liquid and the solution.

Boiling Point

The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure surrounding the liquid. At this point, the liquid transitions to a gas. In the phase diagram, the boiling point can be determined by finding where the vapor pressure curve intersects the 1 atm line, which is crucial for comparing the boiling points of the first and second liquids.

Raoult's Law

Raoult's Law states that the vapor pressure of a solvent in a solution is directly proportional to the mole fraction of the solvent present. When a volatile solute is added to a solvent, the overall vapor pressure of the solution decreases compared to the pure solvent. This principle helps explain why the boiling point of the solution (red curve) is higher than that of the pure solvent (green curve), indicating that the second liquid has a higher boiling point.