= Solution
<DLVO theory> adds short-range attractive <Van der Waals forces> to repulsive overlap of the <electric double layer>. Increasing salt concentration raises the <ionic strength> and shortens the <Debye length>, screening the repulsion. The repulsive barrier falls, so thermal collisions can carry particles into the primary attractive minimum and cause <colloidal coagulation>. An intermediate shallow secondary minimum can instead produce weak, potentially reversible association.
The figure's assignments are \b[a: low salt; b: intermediate salt; c: high salt]. Curve a has the largest protective barrier, b a lower barrier and secondary minimum, and c no effective repulsive barrier. This explanation assumes the surface charging chemistry does not independently reverse or radically change the surface potential.
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