Question concerning solubility equilibriums for salts

[Nikitin]

Hi. Let's say we have a salt AB with a very low Ksp. A bunch of it gets thrown into the water and mixed.

So:

K=[A+][B-]/[AB].

Ksp=[A+][B-]

Why is it OK to remove [AB], then? ie why does [AB] equal 1? I thought [AB] was concentration of the substance in the water, but it is the substance's activity in the water, according to wikipedia?

Please explain this so a high-schooler can understand.

 

[Borek]

[AB] would be concentration of the substance, but the substance is not dissolved.

Imagine you have a single large crystal of a weakly soluble salt, that is a wall of the vessel in which you are dissolving the substance. Concentration of the undissociated substance in the solution is zero, however, it is in the contact with water, so the ions can can be leached into the solution. When you have smaller crystals situation is identical, they form a separate phase.