  Molecular dynamics simulations for aqueous sodium chloride solutions were
carried out at various concentrations. Supplementary to the Debye-H\"uckel
theory, reversible transient nucleation of ions was observed even in dilute
solutions. The average size of formed ion clusters and the lifetime of ion
pairs increase with concentration until the saturation point, when ion clusters
become stable and individual ions adjust their positions to form ordered
lattice structures, leading to irreversible ion crystallization, which is
beyond the description of the classical nucleation theory.
