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Ksp, saturated solutions and precipitation (Ionic Equilibria)

  1. Oct 15, 2011 #1
    I have the following doubts while understanding the concept. Please help me solve them :-
    (1) What is a precipitate exactly? While determining the Ksp values, we say that there in a dynamic equilibrium between the solid undissolved solute in contact with the solution and the ions in solution. Is this solid undissolved solute the 'precipitate'?
    (2) Is there any difference between such solutions and a saturated solution?? If yes, then what? And if not, does this mean that saturated solutions contain precipitate?
    (3) We say that when ionic product > Ksp, the equilibrium is disturbed and the reaction proceeds in the backward direction and precipitation occurs. But isn't there a precipitate (undissolved solid solute) at the above equilibrium? Then why this 'other' precipitation?
  2. jcsd
  3. Oct 15, 2011 #2


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    Staff: Mentor

    Yes, solid is a precipitate.

    If you mix two solutions containing ions that form a weakly soluble salt, as loin gas their ionic product is below Ksp nothing happens. Then moment the ionic product raises up to Ksp, precipitate appears. When you mix two concentrated solutions quickly, as every reaction has a finite speed, there is a short moment when ionic product is above Ksp and there is no precipitate yet - but it will usually appear very fast. There is a kind of initial energy barrier (google for nucleation) that has to be overcomed, so it may happen that supersaturated solution can be quite stable - but it is stable only in kinetic terms, not thermodynamically.
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