Henry’s law can be understood qualitatively in terms of the kinetic molecular
theory. The amount of gas that will dissolve in a solvent depends on how frequently the gas molecules collide with the liquid surface and become trapped by the condensed
phase. Suppose we have a gas in dynamic equilibrium with a solution [Figure 12.6(a)].
At every instant, the number of gas molecules entering the solution is equal to the
number of dissolved molecules moving into the gas phase. If the partial pressure of the gas is increased [Figure 12.6(b)], more molecules dissolve in the liquid because more molecules are striking the surface of the liquid. This process continues until the concentration of the solution is again such that the number of molecules leaving the solution per second equals the number entering the solution. Because of the higher concentration of molecules in both the gas and solution phases, this number is greater in (b) than in (a), where the partial pressure is lower.