Evaporation speeds up when the liquid’s vapor pressure exceeds the partial pressure of that vapor in the surrounding air, creating a pressure gradient that pushes molecules into the gas phase.
What vapor pressure really is
Vapor pressure is the pressure exerted by a vapor when a liquid and its gas are in equilibrium at a fixed temperature. It reflects how readily molecules escape the liquid surface. Substances with weak intermolecular forces—like acetone—have high vapor pressures, while those with strong forces, like water, have low vapor pressures at the same temperature.
How the pressure difference drives evaporation
In an open system the air above the liquid already contains some vapor. Evaporation continues only while the vapor pressure at the liquid surface (Ps) is higher than the ambient vapor pressure (Pa). The net flux J of molecules is roughly proportional to this difference:
J = k · (Ps – Pa)
k is a mass‑transfer coefficient that depends on airflow, turbulence, and surface area. When Pa approaches Ps, the gradient shrinks and the evaporation rate drops.
Temperature’s role
Temperature raises the average kinetic energy of molecules, shifting the Maxwell‑Boltzmann distribution toward higher energies. Consequently, more molecules can overcome intermolecular attractions, and vapor pressure climbs exponentially. The Clausius‑Clapeyron relation describes this rise:
ln(P2/P1) = –ΔHvap/R · (1/T2 – 1/T1)
Higher temperature therefore boosts both Ps and the pressure gradient, speeding up evaporation.
Vapor‑pressure deficit (VPD) in everyday situations
VPD is the difference between the saturation vapor pressure at the liquid’s temperature and the actual vapor pressure in the surrounding air. A high VPD means the air can accept many more molecules, so evaporation is rapid, think of a dry day drying a wet sidewalk. A low VPD, such as on a humid morning, limits the rate because the air is already near saturation.
Evaporation versus boiling
Both processes involve vapor pressure, but they occur under different conditions. Evaporation is a surface phenomenon that happens at any temperature as long as Ps > Pa. Boiling occurs when Ps equals the external atmospheric pressure, allowing vapor bubbles to form throughout the liquid.
Practical implications
- In cooling towers, raising water temperature increases vapor pressure and enhances heat removal.
- Paints and inks are formulated with solvents that have high vapor pressures for quick drying.
- Food storage aims to keep humidity low, increasing VPD and reducing spoilage from excess moisture.