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.