Solids and Liquids and the K expression

[Michelle Fu 1H]

Why aren't solids and liquids included in the K expression? I didn't completely understand Dr. Lavelle's explanation.

[Rami_Z_AbuQubo_2K]

Solids and liquids are not included in the K expression because there is no such thing as a concentration of a solids. For liquids, the molar concentration of a pure substance, like water, does not change in a reaction. The amount of liquid that reacts in a reaction is so small that both amounts of the solvent in the reactants and products will be the same. When you input this into the ratio, they will essentially cancel out and not be of any real use to the value of Kc.

[Miriam Sheetz 2B]

Pure solids or liquids are excluded from the equilibrium expression because their concentrations stay the same during the reaction. The density of a pure liquid or solid is the same, regardless of how much pure liquid or solid is present.