In a mixture of ideal gases, each gas has a partial pressure which is the pressure that the gas would have if it alone occupied the volume of the mixture at the same temperature. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture.
The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. Gases dissolve, diffuse, and react according to their partial pressures, and not according to their concentrations in gas mixtures or liquids.
This general property of gases is also true of chemical reactions of gases in biology. For example, the necessary amount of oxygen for human respiration, and the amount that is toxic, is set by the partial pressure of oxygen alone. This is true across a very wide range of different concentrations of oxygen present in various inhaled breathing gases, or dissolved in blood.
Read more about Partial Pressure: Dalton's Law of Partial Pressures, Ideal Gas Mixtures, Partial Volume (Amagat's Law of Additive Volume), Vapor Pressure, Equilibrium Constants of Reactions Involving Gas Mixtures, Henry's Law and The Solubility of Gases, Partial Pressure in Diving Breathing Gases, In Medicine
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