The Pressure to Evaporate: A Guide to the Vapor Pressure Calculator

Vapor pressure is the pressure exerted by the vapor of a substance when it is in thermodynamic equilibrium with its condensed phase (liquid or solid). In simpler terms, it's a measure of a liquid's tendency to evaporate. At any given temperature, molecules on the surface of a liquid can escape into the gas phase. These gas molecules create a pressure above the liquid, and this is the vapor pressure. A substance with a high vapor pressure at normal temperatures is considered volatile (e.g., gasoline, rubbing alcohol). Vapor pressure is highly dependent on temperature; as temperature increases, more molecules gain enough kinetic energy to escape the liquid phase, and the vapor pressure rises.

The relationship between vapor pressure and temperature is not linear but exponential. The Clausius-Clapeyron equation is a fundamental thermodynamic relation that describes this relationship. This calculator uses a two-point form of the equation to estimate the vapor pressure of a liquid at a new temperature (T₂), given a known vapor pressure at a reference temperature (T₁) and the substance's enthalpy of vaporization (ΔHᵥₐₚ). This is an incredibly powerful tool for chemists and engineers, as it allows them to predict a liquid's boiling point at different atmospheric pressures or to estimate its volatility under various conditions, which is crucial for distillation processes, safety assessments, and chemical engineering design.

The Clausius-Clapeyron Equation (Two-Point Form)

The equation used by this calculator is:

ln(P₂/P₁) = - (ΔHᵥₐₚ / R) * (1/T₂ - 1/T₁)

Where:

P₁ and T₁ are a known vapor pressure and its corresponding absolute temperature. A common reference point is the normal boiling point, where P₁ = 1 atm.
P₂ and T₂ are the vapor pressure and absolute temperature at a new set of conditions.
ΔHᵥₐₚ is the enthalpy of vaporization of the substance (the energy required to turn it from a liquid to a gas), typically in J/mol or kJ/mol.
R is the ideal gas constant, 8.314 J/(mol·K).

It is essential that the temperatures (T₁ and T₂) are in the absolute scale (Kelvin) for this equation to be accurate.

Boiling Point and Vapor Pressure

A liquid's boiling point is defined as the temperature at which its vapor pressure equals the surrounding atmospheric pressure. At sea level, the atmospheric pressure is 1 atm. For water, this occurs at 100°C (373.15 K). However, at a higher altitude, like in Denver, the atmospheric pressure is lower. Because the atmospheric pressure is lower, water's vapor pressure can equal it at a lower temperature, which is why water boils at about 95°C in Denver. The Clausius-Clapeyron equation can be used to predict exactly this kind of behavior.

Vapor Pressure Calculator