Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. It is a measure of the tendency of particles to escape from the liquid or solid state into the gaseous phase. A substance with a high vapor pressure at normal temperatures is often referred to as volatile.

The vapor pressure of a liquid increases with temperature because, at higher temperatures, more molecules have enough kinetic energy to escape from the liquid surface. When the vapor pressure equals the external pressure (typically atmospheric pressure), the liquid boils. This is why water boils at a lower temperature at high altitudes where atmospheric pressure is lower.

The Antoine equation is a semi-empirical correlation that describes the relationship between vapor pressure and temperature for pure substances. Developed by Louis Charles Antoine in 1888, it is derived from the Clausius-Clapeyron equation and provides accurate predictions within specific temperature ranges.

The equation uses three substance-specific constants (A, B, and C) that are determined experimentally. These constants are tabulated for thousands of compounds and are widely used in chemical engineering calculations, distillation design, and thermodynamic modeling. The accuracy of the Antoine equation depends on using it within the recommended temperature range for each substance.

Understanding vapor pressure is crucial in many fields and applications:

• Distillation: Separation of liquid mixtures based on different vapor pressures
• Evaporative Cooling: Design of cooling systems and air conditioning
• Fuel Systems: Understanding gasoline volatility for engine performance
• Pharmaceuticals: Drug formulation and stability studies
• Environmental Science: Predicting evaporation rates and air pollution
• Food Processing: Freeze-drying and vacuum evaporation processes
• Meteorology: Understanding humidity and dew point calculations