We turn now from molecules and phases to the domain of surfaces — the domain of interfacial phenomena and of colloid chemistry. When we considered molecules on Level I, we thought of them as being exposed to similar molecules on all sides; when we considered phases on Level II, we thought of them as surrounded by more of the same phase or by associated phases. But now we come to the situation where some of the molecules which comprise the material are exposed to different molecules — alien molecules of gas, or liquid, or even solid. The boundary is traditionally called a surface if it separates a condensed phase from the air, and an interface if it separates the condensed phase from a liquid or a solid, but the same principles are involved in both situations. As before, we must consider the surface or interface not only in terms of structure and composition, but also in terms of morphology, condition, and behavior.