Predicting Whether Precipitate Will Form Or Not
1. Q = the product of the ion concentrations which actually exist in solution = “what we have”
2. Ksp = the product of ion concentrations required to establish a solubility equilibrium = “what we need to form a saturated solution”
3. Q ＜ Ksp
In this case “what we have” is less than “what we need to form a saturated solution” so that there are not enough ions in solution to form a precipitate.
conclusion: A precipitate cannot form if Q ＜ Ksp
4. Q = Ksp
In this case “what we have” is just equal to “what we need to form a saturated solution”. The minimum possible amount of precipitate forms at this point.
Conclusion: A barely saturated solution is formed if Q = Ksp
5. Q ＞ Ksp
In this case “what we have” is greater than “what we need to form a saturated solution” and there is an excess of ions, over and above the amount needed to form a precipitate. Therefore, a precipitate must form and will continue to form until the excess ions have been removed from solution and the value of Q has been reduced down to the value of Ksp.
Conclusion: A precipitate will form if Q ＞ Ksp.
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