fugacity
/ fyoo-GASS-it-ee /
Fugacity is the 'effective pressure' of a real gas — the pressure it would need to have if it behaved like a perfect, ideal gas while keeping exactly the same chemical tendency to escape. Real gases, especially when squeezed hard or chilled, don't quite follow the neat ideal-gas rules because their molecules attract and jostle one another. Fugacity is the honest fix-up that lets you keep using the elegant ideal-gas formulas anyway.
Precisely, fugacity is the pressure-like quantity you must substitute for true pressure so that the simple expression for a gas's chemical potential stays correct for a real gas. Its name comes from a Latin root meaning 'fleetingness' or 'tendency to flee,' because it measures a substance's escaping tendency. The ratio of fugacity to actual pressure, called the fugacity coefficient, tells you how far the gas departs from ideal behavior — it equals one for a perfect gas.
Why it matters: fugacity lets all the clean thermodynamic machinery built for ideal gases carry over to the messy real gases of industry, deep wells, and high-pressure chemistry. The honest caveat: it is a calculated, model-dependent quantity, not something you read straight off a gauge — and its liquid-and-solution cousin, activity, plays the same correcting role for concentrations in mixtures.
Nitrogen at 1 bar has a fugacity almost equal to 1 bar — nearly ideal. Compress it to hundreds of bar and its fugacity drifts well away from the gauge pressure, which is why high-pressure ammonia plants must reckon with fugacity, not raw pressure.
Near ideal, fugacity ≈ pressure; under extreme conditions they part ways.
Fugacity has units of pressure but is not a pressure you could measure with a barometer — it's a thermodynamic stand-in. The concept was introduced by G. N. Lewis, who also gave us activity, its counterpart for solutions.