 Although thermodynamics provided no sharp distinction between compounds and solutions, it did allow the formulation of a concept for a special case called an ideal solution. An ideal solution forms because its increased stability compared with the separated components is entirely due to the entropy of mixing. This can be understood as a precisification of the idea of a purely mechanical mixture. In contrast, compounds were stabilized by interactions between their constituent components over and above the entropy of mixing. For example, solid sodium chloride is stabilized by the interactions of sodium and chlorine, which react to form sodium chloride. The behavior of real solutions could be compared with that of an ideal solution, and it turned out that non-ideality was the rule rather than the exception. Ideality is approached only in certain dilute binary solutions. More often, solutions exhibited behavior which could only be understood in terms of significant chemical interactions between the components, of the sort characteristic of chemical combination.