In by book, under the topic Heterogeneous Equilibrium it is given
For pure solid and pure liquid, active mass is taken to be unity, i.e., 1 as they remain constant throughout the reaction.
Could you please explain what are the pure solids and liquids being mentioned here?
I read some related answers to similar question but none addresses which compounds are pure which are not. Or, if I haven't noticed properly, kindly give me the link to clarify my doubt.