Chemical Equilibrium. Think of equilibrium as a state of balance. Imagine a revolving door with the same number of people entering at the same rate as exiting . This is equilibrium.
Ref: Merrill Chemistry Textbook
Notice the phase of the reactants and product. They are all gases. We can calculate concentrations of gases. We also know that we can change the concentrations of gases during the reaction.
CaCO3(s)↔ CaO(s) + CO2(g)
Notice that only CO2 is a gas. The other two substances are solids. The concentration of a solid will not change. Thus, we enter 1 in the Kc equation.
Example: N2(g) + 3H2(g)↔ 2NH3(g)