Final answer:
During the fixing step of automatic film processing, the unexposed silver halide crystals are removed when they react with sodium thiosulfate, forming a complex ion that is washed away.
Step-by-step explanation:
The unexposed silver halide crystals are removed from the film base during the fixing step of automatic film processing. This happens when the unexposed silver halides react with sodium thiosulfate (Na2S2O3, also known as hypo). The reaction produces a complex ion, Ag(S₂O₃)₂³−, which is soluble and can be washed away from the film, leaving behind the reduced silver that forms the image. This reaction is part of the multiple equilibria processes in film development, ensuring that the image remains stable and clear.