Vapor Pressure and the Clausius-Clapeyron Equation

Every liquid exerts a vapor pressure set by the balance between molecules escaping into the gas phase and returning. The Clausius-Clapeyron equation ln(P2/P1) = −(ΔHvap/R)(1/T2 − 1/T1) predicts exactly how that pressure grows with temperature — a small change in temperature can multiply the pressure many times over.

The Boiling Connection

Boiling is simply the moment a liquid's vapor pressure reaches the external pressure — 101.3 kPa at sea level, less at altitude. With water's defaults the two-point equation gives about 86 kPa at 373 K; the textbook boiling point of 373.15 K is recovered when ΔHvap is allowed to vary with temperature. Over small ranges — a few tens of kelvins — the constant-ΔH approximation is accurate to a few percent, which is why it remains the workhorse of vapor-pressure engineering.

What ΔHvap Tells Us

The enthalpy of vaporization measures how strongly molecules hold onto each other. Water's 40.7 kJ/mol reflects its hydrogen bonding; methane's 8.2 kJ/mol barely holds its molecules together. Higher ΔHvap means a steeper curve — and a liquid that resists boiling until much higher temperatures.