An equilibrium mixture of two solids and a gas, in the reaction$\mathbf{\text{XY(s)}}\mathbf{\rightleftharpoons }\mathbf{\text{X(g)+Y(s)}}$, is depicted at right (X is green and Y is black). Does scene A, B, or C best represent the system at equilibrium after two formula units of Y(s) is added? Explain.

Le-Chatelier's principle states that, whenever a system at equilibrium is disturbed, the system will undergo reactions and try to cancel that effect and retain equilibrium. Changes in concentration of any component, temperature, pressure, or volume are all examples of disturbances.

Considering the given information,

$\mathbf{\text{XY(s)}}\mathbf{\rightleftarrows }\mathbf{\text{X(g)+Y(s)}}$

Because Y is a pure solid, its concentration is usually assumed to be unity. The addition of Y has no effect on equilibrium position as long as some Y is present. Scene A is the best representation of the system at equilibrium after adding two units of Y. That is, even after the addition of Y, the concentration of XY does not change.

Therefore, the scene A is best represent the system at equilibrium after two formula units of Y(s) is added.