A sealed pop bottle reaches equilibrium when the pressure of carbon dioxide gas in the bottle is balanced with the pressure of the gas dissolved in the liquid. This creates a stable state where gas going out of the liquid equals gas being absorbed. However, an opened bottle is not in equilibrium because the sudden drop in pressure causes more gas to escape, disrupting the balance and preventing equilibrium.

Pure solids and liquids are left out of the equilibrium constant expression for heterogeneous equilibria because their concentrations don't change during a reaction, so including them wouldn't affect the equilibrium constant's value or prediction accuracy.