Solubility Product Constant (Ksp)

Marks the thermodynamic boundary where the dissolved state and the solid state balance. It is defined in terms of ion activity, not concentration. Crossing Ksp grants permission for scale but does not by itself cause a deposit, which depends on kinetics.

$$\text{Below: dissolved favored}\mid \text{At }{K}_{sp}:\text{balance}\mid \text{Above: precipitation favored}$$

Variables

Symbol	Meaning	Unit
$K_{sp}$	Solubility product constant at which dissolution and precipitation are in equilibrium on average	unitless (activity based)

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Part of The Blueprint of Water