What mixtures can be separated by precipitation?
There are two meanings to the word percipitation.
In chemistry, when two solutions are mix together, a precipitate is formed if the product is not soluble. In other words, it's a creation of a solid inside a solution.
For example, the mixture of silver nitrate and silver chloride causes a precipitate to be form.
The knowledge of Ksp values allows you to predict if a precipitate is likely to form. The solubility constant, Ksp, has a fixed value at a given temperature and is independent of the concentration of the individual ions.
In earth science, precipitation means water that falls back to the earth surface in the form of either rain, hail, snow or sleet.