  Phase separation is the thermodynamic process that explains how droplets form
in multicomponent fluids. These droplets can provide controlled compartments to
localize chemical reactions, and reactions can also affect the droplets'
dynamics. This review focuses on the tight interplay between phase separation
and chemical reactions originating from thermodynamic constraints. In
particular, simple mass action kinetics cannot describe chemical reactions
since phase separation requires non-ideal fluids. Instead, thermodynamics
implies that passive chemical reactions reduce the complexity of phase diagrams
and provide only limited control over the system's behavior. However, driven
chemical reactions, which use external energy input to create spatial fluxes,
can circumvent thermodynamic constraints. Such active systems can suppress the
typical droplet coarsening, control droplet size, and localize droplets. This
review provides an extensible framework for describing active chemical
reactions in phase separating systems, which forms a basis for improving
control in technical applications and understanding self-organized structures
in biological cells.
