Undissolved KNO3 crystals sit at the bottom of a saturated aqueous solution held at constant temperature. What is occurring at the particle level?

Core Concept

Saturation is a dynamic equilibrium rather than a standstill. At every exposed crystal face, water molecules detach K+ and NO3- ions into solution exactly as fast as dissolved ions return to the solid. Because the opposing rates match, the dissolved concentration rests at the solubility limit and the mass of undissolved solid stays fixed. Adding more solid accomplishes nothing until temperature changes and shifts the balance.