Why is a saturated salt solution at equilibrium?
Equilibrium means that there are two opposing processes occurring at the same time at equal rates.
For example, imagine you add enough salt into a beaker containing 100mL of water so that no more salt will dissolve and some salt is visible in the solid state at the bottom of the beaker.
Individual Na+ and Cl− ions may transition back and forth between the dissolved state and the solid state. The two processes shown below are both happening at the same time at equal rate.
NaCl(s) -> Na+(aq) + Cl−(aq)
Na+(aq) + Cl−(aq) -> NaCl(s)
Watch the video:
video from: Noel Pauller
Try the simulation available here: http://phet.colorado.edu/en/simulation/soluble-salts
Use the salt shaker to add salt to the point where you see some of the Na and Cl ions remain bonded together. As time goes by, you will see some of the ions leave the salt crystal (solid NaCl) while other ions that were free in solution will bond onto the crystal.