Concept:Chemical equilibrium is dynamic and can respond to external changes by shifting to restore balance.
Explanation:In a chemical system at equilibrium, the forward and reverse reactions continue at equal rates.
Therefore, the reaction is reversible and does not go to completion.
When a stress such as a change in concentration, pressure, or temperature is applied, the system shifts in the direction that reduces that stress.
In this way, equilibrium is re-established under the new conditions.
A catalyst only speeds up the time taken to reach equilibrium.
It does not change the position of equilibrium.
Thus, the only correct statement is that equilibrium is re-established when a stress is applied.
Answer:C. equilibrium is re-established when a stress is applied.