A statement for an ideal mixture of nonreacting gases that the total pressure exerted by the mixture equals the sum of the partial pressures of each component gas, where a component's partial pressure is the pressure it would exert alone at the same temperature and volume.

Principle

Independent contributions to total pressure: in the ideal approximation each species behaves independently and contributes a pressure proportional to its mole fraction, so P_total = Σ P_i = (Σ n_i)RT/V or equivalently P_i = x_i P_total.

Demonstration

Collecting gas over water and measuring total pressure, one subtracts the known vapor pressure of water to obtain the partial pressure of the collected gas; in air at sea level, oxygen partial pressure ≈ 0.21·P_total gives the pressure contribution of oxygen.

Misapplication

Applying simple partial-pressure additivity to reacting or strongly interacting mixtures where species associate, dissociate, adsorb, or where non-ideal fugacity corrections are required; assuming volume fractions equal mole fractions in compressible, non-ideal regimes.

Consequence

Allows decomposition of measured total pressure into component contributions, enabling determination of composition from pressure measurements, calculation of equilibrium constants in gas-phase reactions under ideal assumptions, and practical use in respiration and gas-supply calculations.

Reversal

The conceptual reversal is to replace additive partial pressures by non-additive effective pressures (fugacities) in non-ideal mixtures; refusing additivity indicates interactions or reactions that invalidate simple mole-fraction scaling.

Boundary

Valid for mixtures of ideal, non-reacting gases in the classical regime; breaks down when chemical reactions change component counts, when significant intermolecular interactions exist, at high pressures or near condensation, and when surface adsorption alters effective concentrations.

Semantic Tension

Tension between the intuitive, additive picture of independent gas pressures and the thermodynamically correct treatment using fugacity and activity in non-ideal or reactive systems; also between partial pressure and Dalton's historical operational measurement practices.

Synthesis

Dalton's law asserts that, for an ideal nonreacting gas mixture, total pressure equals the sum of individual gases' partial pressures because each species contributes independently in proportion to its mole fraction; departures flag interactions, phase change or reaction.