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while writing the equilibrium constant , we generally use aq and gases only.. why do we ignore pure solids and liquids?
In writing the expression for the equilibrium constant, the molar concentrations (or partial pressures) of the product species raised to the power of their stoichiometric coefficients are taken in the numerator while those of the reactant species are placed in the denominator.  However, the following conventions must be kept in mind while writing the expressions for the equilibrium constant. 
For heterogeneous reactions, by convention, the molar concentrations of pure solids and pure liquids are taken as 1 i.e., these are merged into the equilibrium constant.  For example,
 
Here C2H5OH(l) is a pure liquid and its molar concentration is, therefore, taken as 1.
If pure liquid (usually water) as one of the reactants is present in large excess, its concentration remains almost constant during the course of the reaction.  It is, therefore, also taken as 1 or it is also merged into the equilibrium constant.
 
 
Here, only the molar concentration of H2O(l) is taken as 1.
The molar concentration of a substance means the number of moles per unit volume: (moles/volume).  Since the number of moles are related to the mass of the substance, therefore, molar concentration is also related to mass/volume will all know that mass divided by volume is the density of a substance.  Therefore,
  
Since density is an intensive property, it is independent of the amount of the substance.  Therefore, the molar concentration of a pure substance (solid or liquid) is independent of the amount of the substance and it is not considered while writing the expression for the equilibrium constant.  But this is not applicable to the substances in gaseous states or aqueous solutions because there amounts in a given volume can vary. 


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