• A?saturated solution?is a solution that contains the?maximum?amount of dissolved salt
• If a second compound, which has an ion?in common?with the dissolved salt, is added to the saturated solution, the solubility of the salt reduces and a solid?precipitate?will be formed
• This is also known as the?common ion effect
• For example, if a solution of?potassium chloride?(KCl) is added to a?saturated solution of silver chloride?(AgCl) a precipitate of silver chloride will be formed
  • The chloride ion is the common ion
• The?solubility product?can be used to predict whether a precipitate will actually form or not
  • A precipitate will form if the product of the ion concentrations is?greater than?the solubility product (K_sp)

Common ion effect in silver chloride

• When a?KCl?solution is added to a saturated solution of?AgCl, an AgCl precipitate forms
• In a?saturated?AgCl solution, the silver chloride is in equilibrium with its ions

AgCl (s) ? Ag⁺?(aq) + Cl^-?(aq)

• When a solution of potassium chloride is added:
  • Both KCl and AgCl have the common Cl^-?ion
  • There is an increased Cl^-?concentration so the equilibrium position shifts to the left
  • The increase in Cl^-?concentration also means that [Ag⁺?(aq)] [Cl^-(aq)] is?greater?than the?K_sp?for AgCl
  • As a result, the AgCl is?precipitated

The addition of potassium chloride to a saturated solution of silver chloride results in the precipitate of silver chloride

Worked Example: Calculations using the?K_sp?values and the concentration of the common ion

Answer

• Step 1:?Determine the equilibrium reaction of CaSO₄

CaSO₄?(s) ? Ca²⁺?(aq) + SO₄^2-?(aq)

• Step 2:?Write down the equilibrium expression for?K_sp

K_sp?= [Ca²⁺?(aq)] [SO₄^2-?(aq)]

• Step 3:?Determine the concentrations of the ions

There are equal volumes of each solution

This means that the total solution was diluted by a factor of 2

The new concentrations of the Ca²⁺?ion is?halved?

= 5.0 x 10^-4?mol dm^-3

The sulfate ion concentration remains the same as it is a common ion and its concentration is the same in both solutions

• Step 4:?Substitute the values into the expression

Product of the ion concentrations = [Ca²⁺?(aq)] x [SO₄^2-?(aq)]

= (5.0 x 10^-4) x (1.0 x 10^-3)

= 5.0 x 10^-7?mol²?dm^-6

• Step 5:?Determine if a precipitate will form

As the product of the ion concentration (5.0 x 10^-7?mol dm^-3?) is smaller than the?K_sp?value (2.0 x 10^-5?mol²?dm^-6), the CaSO₄?precipitate will?not?be formed