7.2.13 Solubility Product

What is Solubility?

In A Level chemistry, we deal with “sparingly soluble” salts (salts that barely dissolve, like Silver Chloride or Barium Sulfate). Even though they look insoluble, a tiny amount does dissolve to form a saturated solution.

When an undissolved ionic compound is in contact with a saturated solution of its ions, an equilibrium is established:

When undissolved magnesium chloride comes into contact with its ions in a saturated solution, an equilibrium between the salt and ions is established

Solubility Product

Consider the equilibrium when the sparingly soluble salt is added to water:

The equilibrium constant () for this equilibrium is given by:

Since the concentration of the solid is constant (constant density), we can multiply both sides by it to get an expression for the solubility constant, :

Definition: Solubility Product ( )

The Solubility Product () is:

• The product of the concentrations of each ion in a saturated solution of a sparingly soluble salt
• At 298 K
• Raised to the power of their relative concentrations.

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Ionic Product

The Ionic Product () is calculated using the exact same formula as , but using the actual concentrations of ions at a specific moment in time, not necessarily at equilibrium.

Important

• : Describes the maximum possible concentration (Saturation limit).
• : Describes the current concentration.

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Predicting Precipitation

By comparing the value of to the fixed value of , we can predict whether a solid will form.

Condition	State of Solution	Physical Result
	Unsaturated	No precipitate forms. More solid can dissolve.
	Saturated	No precipitate forms, but no more solid can dissolve. Equilibrium is formed.
	Supersaturated	Precipitate forms. The excess ions crash out of solution until drops back down to equal
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