Final answer:
The Debye-Hückel theory posits that ions interact weakly at low concentrations with interionic attractions affecting ion behavior, especially in more concentrated solutions. In dilute solutions, effective ion concentrations match actual concentrations more closely.
Step-by-step explanation:
The Debye-Hückel theory of activity coefficients of electrolyte solutions is based on the assumption that ions interact weakly at low concentrations. The theory suggests that while interionic attraction is greatly reduced in an aqueous solution due to solvation and the insulating effect of the polar solvent, it is not completely eliminated. These residual attractions influence the behavior of the ions, especially at higher concentrations where the interactions become significant. Consequently, the activity or effective concentration of an ion is lower than the actual concentration. In contrast, at extremely dilute solutions, ions are more widely separated, and the effective concentrations closely approximate the actual concentrations, behaving more ideally.
Note that strong electrolytes like HCl dissociate almost completely, creating high concentrations of ions, while weak electrolytes only partially dissociate, forming lower ion concentrations. The van't Hoff factors reflect this phenomenon, only reaching the expected values in dilute solutions.