Why real gas mixtures need a second, corrected pressure — one that actually tracks each component's escaping tendency instead of just its share of the total.
Dalton's Law gives every process engineer a fast, satisfying answer for how much pressure each gas in a mixture is "responsible for": multiply its mole fraction by the total pressure. It's simple, it's intuitive, and for a huge range of everyday conditions it's a perfectly good stand-in for how badly that component "wants" to leave the mixture — condense, react, or cross into another phase. The trouble is that partial pressure is a bookkeeping split of an ideal-gas total. It says nothing, by itself, about the intermolecular forces the real molecules around it are actually exerting. At high pressure, or in mixtures of dissimilar molecules that interact strongly with one another, that gap between the simple split and the real physics stops being ignorable — and process engineers reach for a second, corrected quantity called fugacity to close it.