to how transient phosphorylation by ATP is used in biology. This can be as conceptually complex as operation of a molecular motor, or as simple as assembly of the anhydride state into an aggregate. Regardless of the behavior, the properties of these systems are fundamentally governed by kinetic control, which can lead to responsive behavior not possible in systems at equilibrium. In this activity, the students explore this chemistry from two perspectives using two different benzoic acids, potassium 4-sulfobenzoic acid (KSBA, Scheme 1) and benzoic acid itself (BA). The difference is that the anhydride of KSBA, KSBA-An, remains soluble throughout the experiment, whereas BA-An precipitates at typical experimental concentrations. KSBA and KSBA-An have been previously used as a model acid for 1 H NMR spectroscopy investigations of carbodiimide-driven anhydride formation. Thus, they are well suited to investigation of anhydride hydrolysis kinetics; the concentration of KSBA-An can be conveniently followed using either IR or 1 H NMR spectroscopies. Here, the students use both methods, allowing the techniques to be compared (the experiment could also be modified to use one method alone). Conversely, when BA is used in place of KSBA, the formation of the BA-An precipitate represents a simple example of transient self-assembly, paralleling other (more-sophisticated) examples of transient crystallization. Precipitation is easily observed by the naked eye and, importantly, the effect of varying the amount of fuel can be determined. The students observe how changing the amount of EDC affects both the activation rate (precipitation) and the deactivation rate (redissolution). The outcome is counterintuitive because of the kinetically controlled nature of the system.