fixing
After the film is developed, any unexposed silver bromide must be removed by a process called fixing; otherwise, the entire film would turn black with additional exposure to light. Although silver bromide is insoluble in water, it is soluble in a dilute solution of sodium thiosulfate Na2S2O3; photographers hypo because of the formation of [Ag S2O3 2]3 ions. Thus washing the film with thiosulfate solution dissolves unexposed silver bromide and leaves a.