Vapor Pressure, Boiling Point, and Freezing Point in Chemistry

Understanding Vapor Pressure and Colligative Properties

Colligative Properties and Vapor Pressure

Liquids are composed of molecules in constant motion. Upon reaching the surface, these molecules can acquire enough energy to transition from the liquid to the gaseous state (i.e., become vapor). If they retain sufficient energy, they remain in the gaseous state; otherwise, they return to the liquid state.

Defining Vapor Pressure

When equilibrium is established between a liquid and its vapor in an enclosed space, the pressure exerted by the vapor over the liquid has a specific value at a given temperature. This is known as vapor pressure.

Factors Affecting Vapor Pressure

• Temperature

  An increase in temperature enhances the kinetic energy of molecules, leading to increased molecular motion. This causes more molecules to transition from liquid to gas, meaning the vapor pressure increases due to a greater number of molecules in the vapor state. Conversely, if the temperature decreases, the reduced molecular motion hinders the passage from liquid to gas, thereby decreasing vapor pressure.

• Solute Presence

  The presence of a nonvolatile solute (i.e., a non-electrolyte) such as glucose, urea, or kerosene, occupies the liquid surface. This prevents some solvent molecules from moving into the gas phase, which implies that the vapor pressure of the solution decreases because there are fewer molecules entering the gaseous state compared to the pure solvent.

Raoult's Law

Raoult's Law states that the decrease in the solvent's vapor pressure is proportional to the molar fraction of the solute.

P₀:

Vapor pressure of the pure solvent

P:

Vapor pressure of the solution

n:

Number of moles of solute

N:

Number of moles of solvent

Key Phase Transition Points

Boiling Point

• Practical Definition

  The temperature at which a liquid boils, or the temperature at which the liquid becomes vapor.

• Technical Definition

  The temperature at which the vapor pressure of the liquid becomes equal to the external atmospheric pressure.

How Crystals Form

When the temperature decreases, the molecular kinetic energy also decreases. This reduction in temperature causes a decrease in the distance separating the molecules, leading to increased intermolecular attraction and eventually crystallization.

Freezing Point

The temperature at which the vapor pressure of the liquid becomes equal to the vapor pressure of the solid state.

The difference between the boiling point and freezing point of a solution is a significant characteristic influenced by solute presence.