Substance a definition

By identifying one component with pure substance a definition of the latter is obtained. As Wald put it Phases are chemically identical when they coexist and correspond to the phase rule for one component. All other bodies are chemically different [Wald, 1897, 647f emphasis original]. To highlight that a prototypical pure substance may occur in several phases, we may choose its invariant triple point as its defining feature. In the prototypical case this will be the triple point of solid, liquid, and vapour (as in the case of water), but a substance that is not stable at higher temperatures, thus not forming a gas phase, may still have a characteristic triple point for example polyethylene has a triple point at which one liquid and two crystalline phases are in equilibrium. A special invariant point is the critical point, not further discussed here. ... 

---