top of page

# Vapor Pressure vs. Atmospheric Pressure in Determining Boiling Point.

Setting the Scene: Liquid-Vapor Equilibrium

Every body of liquid is accompanied by a vapor layer, mirroring the liquid's composition. For example, a glass of water contains not just liquid water, but also a subtle sheen of water vapor above.

---

The Push and Pull: Vapor and Atmospheric Forces

1. Vapor Pressure: Emanating from the water vapor molecules is an upward force, termed the vapor pressure.

2. Atmospheric Pressure: The overarching atmosphere presses downward, exerting its atmospheric pressure. This keeps the vapor molecules in check and curtails their escape.

---

The Intricacies of Equilibrium & Balance

In standard conditions, atmospheric pressure reigns, maintaining an equilibrium between the vapor and liquid. However, this balance is preserved only when the vapor pressure is inferior to the atmospheric pressure.

---

Le'Chatlier's Principle & Equilibrium

Central to understanding this balance is Le'Chatlier's principle. The equilibrium constant (Keq) represents the ratio of vapor phase molecules to liquid phase molecules. The vapor pressure is essentially a manifestation of this Keq.

1. Reaction Quotient (Q) & Temperature: As temperature shifts, so does Q. Why? Because of the equation ΔG = ΔH - TΔS. A change in temperature affects the Gibbs free energy (ΔG), which then influences the direction and extent of the equilibrium shift.

2. Entropy's Role: In the ΔG equation, entropy (ΔS) is multiplied by temperature (T). This means the importance of entropy escalates with temperature. Since the gas phase has a higher entropy than the liquid phase, as temperature rises, the system becomes more inclined toward the gas phase. Hence, the vapor pressure escalates.

---

Vapor Pressure vs Atmospheric Pressure in Determining Boiling Point

Warming the system elevates the vapor pressure. As this pressure draws level with or surpasses atmospheric pressure, the stable equilibrium is thrown into disarray.

1. Entropy and Phase Change: Given that the gas phase possesses greater entropy than its liquid counterpart, with increasing temperature, the system favors the vapor phase even more.

2. Disruption of Equilibrium: The bolstered vapor pressure, driven by temperature and entropy changes, disrupts the equilibrium between the vapor and atmosphere. This leads to a net migration from the liquid phase, resulting in boiling.

---

In summation, vapor pressure and atmospheric pressure's tango is more than just a simple push-pull dynamic. It's an intricate dance, directed by principles of thermodynamics and equilibrium, that defines the transition from liquid to vapor.