# Relevance of equilibrium constant Kp for reaction when reactant is solely a solid and is absent

Consider a reaction where a solid dissociates to give gases. There must a equilibrium between them, define the equilibrium constant in terms of pressure as $$K_p$$. Suppose I take the gases alone in a container without the solid. The amount is such that the $$Q_p$$ comes out to be less than $$K_p$$. Now would they show back reaction?

While being introduced to concept of $$K_c$$ we are told concentration of solids is constant so it isn't considered in equilibrium constants. But here the concentration is altogether 0, so why won't/how will we take that into consideration.

• The concentration of the solid is constant but not that of the gases. If you mix gases that can react to form a solid they will do so until the $K_p$ is satisfied. Mar 13, 2022 at 9:28
• Think of ammonium chloride $\ce{NH4Cl}$. It gets dissociated into two gases : $\ce{NH3}$ and $\ce{HCl}$ at high enough temperature. Mar 13, 2022 at 10:26
• @andselisk the answer well explains why concentration of solids is constant, but that too is when the some solid is indeed present, here it is completely absent initially, so wouldn't we observe back reaction? Mar 29, 2022 at 16:58