Final answer:
When sodium chloride is added to a saturated solution, precipitation occurs because the solution has already reached its capacity to dissolve more salt, and additional sodium chloride will form a precipitate without increasing the solubility.
Step-by-step explanation:
When sodium chloride is added to a saturated solution, the solubility of the salt does not increase because the solution has already reached its maximum dissolution capacity at a given temperature. In such a solution, there is a dynamic equilibrium between the salt dissolving and salt crystallizing. Upon adding more sodium chloride to a saturated solution, no net increase in dissolved salt occurs because as quickly as salt molecules enter the solution, they also rejoin the solid phase, maintaining a constant concentration.
Therefore, the correct option to the question "What happens when sodium chloride is added to a saturated solution?" is D) Precipitation occurs. Once a solution is saturated and additional solute is added, any additional solute will not dissolve and will start to settle out of the solution as a precipitate. This physical process is a result of the rate of dissolution and the rate of recrystallization being in equilibrium.