[cmquer]

In equilibrium state, I know that the concentrations of pure solids, pure liquids and solvents in dilute solutions are constant at a given temperature, but I don't know the principle of this.

For example,
CaCO3 (s) <-- --> CaO (s) + CO2 (g)
Kc = [CO2 (g)]

I really confused that how the solid (CaCO3) will be constant. My understanding is: At the beginning, there are only CaCO3 solid but no CaO and CO2, therefore there should be a reduce of CaCO3 conc. to form CaO and CO2.

So how the conc. of CaCO3 be constant?

Thank you for answering.

[Borek]

Not concentration but activity.

[cmquer]

Not concentration but activity.

Could you explain in a more detail way? Thank You.

[Borek]

If there are solids present we refer to their activity, not their concentration. Solid activity is always 1 and amount of the solid doesn't change its activity. It may change reaction kinteics, as the reaction takes place on the surface, thus larger surface means faster reaction, but it won't change equilibrium.

If it seems to you that the whole thing of constant activity of solids is against intuition - don't bother, you are not the first to think that way. But experiments confirm it is 1, so you better refocus your intuition