STPM Chemistry Form 6 Notes – Heterogeneous Ionic Equilibrium (Part 6 – Final)

by BerryBerryTeacher

in Berry Reference (Notes)

Common Ion Effect refers to a the effect when two substances which will give the same ion when ionised are involved in a chemical equilibrium. There will be an effect in the degree of solubility in solution when it happens. Part 6 of Berry Berry Easy’s notes on Heterogeneous Ionic Equilibrium is exactly about the effects of common ion effect and examples will be given to illustrate the effects.

[Tips: The key to understanding the effects of common ion effect is to imagine throwing two packet of sugar in the same cup of tea, and imagine the solubility as if you put the two packets of sugar in separate cups. By answering it correctly, you would have understand the concept of common ion effect.]

STPM Chemistry Form 6 Notes – Heterogeneous Ionic Equilibrium (Part 6 – Final)

Common Ion Effect

Definition of common ion effect –

  • the reduction in solubility of an ionic solid (salt) in solution, because one of the ions in the salt is already present in solution. / the effect of reducing the solubility of a sparingly soluble ionic compound when another compound that contains an ion in common with it is added.
  • the effect of reducing degree of dissociation of a weak electrolyte when another electrolyte with a common ion is added to it.

According to common ion effect, if one of the ions is already present in solution, then less salt is capable of dissociating, so the solubility is reduced.

Heterogeneous Ionic Equilibrium Precipitation

Heterogeneous Ionic Equilibrium Precipitation

Case 1:

AgCl(s) + H2O(l) <—->Ag+(aq) + Cl- (aq)

With the addition of either Ag+ or Cl- to solution in this reaction, there is a reduction on the amount of AgCl that can dissociate into it. This is because the presence of one of the ions constituting (Ag+ or Cl- ) in the solution.


At what pH is Ca(OH)2(s) most soluble in water?

A. 11
B. 9
C. 5
D. 3


  • pH at which Ca(OH)2(s) is most soluble is the pH at which there is the least common ion (OH- ) present in solution.
  • There is the least OH- present at lower pH values, where the solution is acidic.
  • Note that basic salts are most soluble in acidic solutions.

Answer: D. 3


Mercury II cations are most soluble in which of the following solution?

A. 0.10 mol dm-3 NaNO3
B. 0.10 mol dm-3 NaI
C. 0.10 mol dm-3 Na2S
D. 0.10 NaCl


  • Find the relative solubility of Hg(NO3)2, HgI2, HgS and HgCl2
  • Nitrates are infinitely soluble in water.
  • Only nitrate (anions) can form hydrogen bonds with water, so it has the greatest salvation energy of the four anions.
  • Nitrate has the largest of anions, therefore it has the weakest electrostatic forces in its lattice structure.

Answer: A

Finally, the end of Part 6, and also the series of notes on the topic of Heterogeneous Ionic Equilibrium for STPM Form 6 Chemistry students from Berry Berry Easy. We hope you understood what have been presented so far.

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