What is one of the factors that can stress a system at equilibrium?

Changing temperature is indeed a factor that can stress a system at equilibrium, as described by Le Chatelier's principle. According to this principle, if an external change is applied to a system at equilibrium, the system will respond by shifting its position to counteract that change and restore a new equilibrium state. Specifically, in the case of temperature changes, if the temperature of an exothermic reaction is increased, the system will shift to favor the endothermic direction to absorb the extra heat. Conversely, if the temperature is lowered, the system will shift towards the exothermic direction to release heat. This illustrates how temperature fluctuations can affect the concentrations of reactants and products in an equilibrium system.

Introducing a catalyst, on the other hand, does not affect the equilibrium position of a reaction; it merely speeds up the rate at which equilibrium is reached without favoring either side of the reaction. Similarly, while reducing the volume of the system can influence gas-phase equilibria by favoring the side with fewer moles of gas, it is not universally applicable to all systems at equilibrium. Thus, changing temperature stands out as a definitive stress factor according to the principles governing chemical equilibria.