Consequently, we introduce an explicit variable to represent the degree of advancement of a process, a progress variable "ξ" for the "extent of reaction" (Prigogine & Defay, p. 18; Prigogine, pp. 4–7; Guggenheim, p. 37.62), and to the use of the partial derivative ∂"G"/∂"ξ" (in place of the widely used "Δ"G"", since the quantity at issue is not a finite change). The result is an understandable expression for the dependence of d"G" on chemical reactions (or other processes). If there is just one reaction