We have seen that ions in aqueous solution are stabilized by the attractions between the ions and the water molecules. why then do some pairs of ions in solution form precipitates?

For a dissolving process, the intermolecular forces of attraction are taken into consideration. There are 3 forces of attraction involved in this process, attraction between solute-solute molecules, attraction between solvent-solvent molecules and attraction between solute-solvent molecules.

When a solute gets dissolved in solvent , there must be a break of solute-solute forces and solvent-solvent forces because then only solute-solvent forces comes into play. The forces of attraction between solute-solvent molecules must be greater than than the individual attraction between two molecules of solute and solvent.

The precipitate forms when the attraction between solute-solvent molecules is less than the solute-solute and solvent-solvent molecules.

aquifers are bounded above and below by impermeable rock. the driving force of groundwater movement in confined aquifers is pressure, rather than gravity.