Antoine Equation: log₁₀(P) = A − B / (C + T)
Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid (or solid) phase. It increases exponentially with temperature. The Antoine equation log₁₀(P) = A − B/(C+T) provides accurate vapor pressure estimates with three substance-specific constants. Water: VP at 20°C = 17.5 mmHg, at 100°C = 760 mmHg (boiling point at 1 atm). The Clausius-Clapeyron equation ln(P₂/P₁) = −ΔHvap/R × (1/T₂ − 1/T₁) provides a simpler two-point estimate. Vapor pressure determines: boiling points, evaporation rates, relative humidity, vapor-liquid equilibrium in distillation, and environmental fate of volatile chemicals. Low VP compounds (like mercury, VP = 0.002 mmHg at 25°C) still pose inhalation hazards.