What is the common ion effect and how does it affect chemical equilibria?

The common ion effect is when the addition of a common ion to a solution reduces the solubility of a salt.

When a salt is dissolved in water, it dissociates into its constituent ions. For example, when sodium chloride (NaCl) is dissolved in water, it dissociates into Na+ and Cl- ions. If a solution already contains one of these ions, then the solubility of the salt will be reduced. This is because the equilibrium between the undissolved salt and its constituent ions will shift towards the undissolved salt, in order to maintain a constant concentration of the ions in solution.

For example, if a solution contains a high concentration of Na+ ions, and NaCl is added to the solution, then the equilibrium between NaCl and its constituent ions will shift towards the undissolved NaCl. This means that less NaCl will dissolve in the solution, as the excess Na+ ions already in solution will combine with the Cl- ions from the NaCl to form undissolved NaCl.

The common ion effect can also affect acid-base equilibria. For example, if a weak acid is dissolved in a solution containing its conjugate base, then the equilibrium between the acid and its conjugate base will shift towards the undissociated acid, reducing the acidity of the solution.

Overall, the common ion effect can have a significant impact on chemical equilibria, and must be taken into account when predicting the behaviour of solutions.