Question

Explain why a solution has a lower boiling point than that of the pure solvent.

Short answer

In conclusion, a solution has a lower boiling point than that of a pure solvent because the presence of a solute disrupts the solvent's vapor pressure and decreases it. The solute-solvent interactions result in fewer solvent molecules being available at the surface to escape into the vapor phase. According to Raoult's Law, this leads to a lower vapor pressure for the solution, which in turn requires a higher temperature to reach the boiling point.

Step-by-step solution

Understand vapor pressure and boiling point

Vapor pressure is the pressure exerted by the vapor molecules of a liquid when they are in equilibrium with the liquid phase. In other words, it is the pressure at which an equal number of molecules are evaporating and condensing. Boiling occurs when the vapor pressure of a liquid becomes equal to the atmospheric pressure. As the temperature increases, the vapor pressure of a liquid also increases until it reaches the boiling point.

Introduce solute-solvent interactions

When a solute is added to a solvent, it interacts with the solvent molecules. The solute-solvent interactions result in the solute molecules occupying some of the space at the surface of the liquid that was previously occupied by the solvent molecules. This results in fewer solvent molecules being available at the surface to escape into the vapor phase.

Raoult's Law and lowering of vapor pressure

Raoult's Law states that the vapor pressure of a solution is equal to the product of the mole fraction of the solvent in the solution and the vapor pressure of the pure solvent. The mole fraction of the solvent in a solution will be less than 1, as the solution contains both solvent and solute molecules. Therefore, the vapor pressure of the solution will be lower than that of the pure solvent.

Lower vapor pressure leads to a lower boiling point

Since the vapor pressure of the solution is lower than that of the pure solvent, it requires more heat and thus a higher temperature to reach the atmospheric pressure, which is the boiling point. Therefore, a solution has a lower boiling point than that of the pure solvent. In conclusion, the presence of a solute in a solvent disrupts the solvent's vapor pressure and decreases it, leading to the requirement of a higher temperature to reach the boiling point. This is why a solution has a lower boiling point than that of a pure solvent.

Key concepts

Vapor Pressure

Vapor pressure is a fundamental concept in understanding why solutions behave differently than their pure solvents. It refers to the pressure created by the vapor of a liquid in equilibrium with its liquid phase. Imagine a closed container with some liquid inside. In this scenario, molecules from the liquid will evaporate, creating vapor above the surface. This vapor reaches a state of equilibrium when the evaporation rate equals the condensation rate. At this point, the pressure of the vapor is termed as the vapor pressure.

Every liquid has a characteristic vapor pressure at a given temperature. When the surrounding atmospheric pressure matches the vapor pressure, the liquid starts to boil. Therefore, the boiling point of a liquid is the temperature at which its vapor pressure equals the ambient atmospheric pressure. Understanding vapor pressure is key to grasping why solutions have different boiling points compared to their pure solvents.

Raoult's Law

Raoult's Law provides insight into how the presence of solute in a solution affects its properties. According to this law, the vapor pressure of a solution is directly proportional to the mole fraction of the solvent in the solution. In other words:

• The vapor pressure of a solution (\( P_{ ext{solution}} \)) = The vapor pressure of the pure solvent (\( P_{ ext{solvent}} \)) multiplied by the mole fraction of the solvent (\( X_{ ext{solvent}} \)).

Since adding solute to a solvent reduces the mole fraction of the solvent (\( X_{ ext{solvent}} \< 1 \)), the vapor pressure of the solution is lower than that of the pure solvent.

Having a lower vapor pressure means that a solution requires a higher temperature to match the atmospheric pressure and reach the boiling point. Therefore, understanding Raoult's Law helps explain why solutions typically have altered boiling points compared to pure solvents.

Solute-Solvent Interactions

Solute-solvent interactions are crucial in affecting the properties of a solution, particularly its vapor pressure and boiling point. When a solute is dissolved in a solvent, these two types of molecules mix at a molecular level. Solute molecules occupy positions that were previously the domain of solvent molecules at the surface of the liquid.

This interaction causes fewer solvent molecules to escape into the vapor phase. As a result, the vapor pressure of the solution becomes lower than that of the pure solvent.

• Effect on boiling point: Due to the decrease in vapor pressure, it takes more energy (heat) for the solvent molecules in the solution to reach the boiling point where the vapor pressure equals atmospheric pressure.

In essence, these interactions are a major reason why the presence of a solute lowers the observed boiling point compared to a pure solvent.