Gas Solubility

Gas solubility in liquids forms liquid-gas solutions, such as soft drinks, where carbon dioxide is dissolved in water, and the ocean, where the solubility of oxygen and carbon dioxide supports marine life. The ability of oceans to dissolve gases impacts weather conditions in the troposphere.

However, gas-liquid interactions vary. For instance, hydrogen chloride gas is highly soluble in water, while oxygen's solubility is much lower. Because these solutions are non-ideal, Raoult’s law, which relates vapor pressure to composition, doesn't apply accurately across all mole fractions.

At low mole fractions, vapor pressure is proportional to the mole fraction, a relationship defined by Henry's law. The proportionality constant in Henry’s law, known as the Henry’s law constant (K_i), depends on the components and temperature. Unlike Raoult’s law, which defines the proportionality constant as the pure component's vapor pressure, Henry’s law uses an experimentally determined value.

Gases that are sparingly soluble in a liquid typically have a low concentration in the solution, making it ideally dilute. In such conditions, Henry's law is applicable. For example, when plotting the mole fraction of dissolved N₂ (or H₂) in water at 50°C against its partial pressure, the plot for N₂ up to 100 atm obeys Henry’s law and is essentially linear. Above 100 atm, the plot shows increasing deviations due to non-ideal gas behavior and pressure dependence on Henry's law constant (K_i). However, H₂ follows Henry’s law up to 200 atm.

At low solute concentrations where Henry’s law applies, the solute’s molality and molar concentration are each essentially proportional to its mole fraction. Hence, Henry's law can use molalities or concentrations instead of mole fractions. The larger the K_i value, the smaller the solubility of the gas.

The solubility of most nonpolar gases in water generally decreases as temperature increases at constant pressure, because dissolution is typically exothermic. Near the critical temperature of water (374 ℃), the concept of gas solubility in liquid water is no longer meaningful because liquid water ceases to exist as a distinct phase.

However, Henry’s law does not apply to all solutions, such as a dilute aqueous HCl, since it dissociates completely in water. Even at high dilution, the behavior of the electrolytes is non-ideal due to ion-ion interactions and deviates from the assumptions deriving from Henry’s law.