Fig. 2.12. Dependency of dynamic viscosity of water and air on temperature
(after, Lu and Likos, 2004)

It was, so far, elucidated the propeties of different phases in soil matrix when each phase is pure. However, each of these phases is by no means pure. The air phase may consist of water vapor and the presence of vapor depoends on several atmospheric state variables. On the other hand, the air and solids may present in the water phase in the form of dissolved air and dissolved salts. Therefore, it is important to study the significance of relative presence of air, water, and solids in the other phases. Some of the important state variables are presented in the following section to explain the equilibrium between water and air phase for the co-existance.