# How does it affect the equilibrium expression if the concentration of any component remains the same?

I am a high school student and I am very confused in Equilibrium expression, My confusion is that "Why we don't write concentration of solids and pure liquids in equilibrium expression?" Many people says that its because their concentration don't changes but how does it related to this? If concentration is same for them then what's the issue in it? I am not able to picturize how does it affect the equilibrium expression if their concentration remain the same?

please Help me regarding this using simple language that I can understand because I am at high school level. If anyone could help me to picturize this thing in my mind then it would be very helpful.

• Well, it doesn't, so it can be ignored, as simple as that. Nov 10, 2020 at 15:20
• Well, you can either define an equilibrium constant with the formal constant concentration of a pure condensed phase in mol/dm3, or you can implicitly involve them in the equilibrium constant value, pretending their concentration is 1 mol/dm3. Why way is more convenient ? The same for the pressure variant of the equilibrium constant. Nov 10, 2020 at 15:22
• why we have to pretend that their concentration is 1mol/dm^3,.it can be of any value.. Nov 10, 2020 at 15:28
• We do not have to. Neither we have to go along stairs forwards. We can go backwards as well. Multiplication or division by 1 is very convenient. You can omit it totally. Nov 10, 2020 at 15:35

Suppose you want to study the equilibrium of the reaction of a metal like zinc $$\ce{Zn}$$ with water to produce $$\ce{ZnO(s)}$$ and $$\ce{H2}$$ :$$\ce{Zn(s) + H2O(g) <=> ZnO(s) + H2(g)}$$ At high temperature, the equilibrium constant may be written $$K_\mathrm{1}=\frac{[\ce{ZnO}]·p\ce{(H2)}}{[\ce{Zn}]·p\ce{(H2O)}}$$ In this expression three parameters are not changing : the constant $$K_\mathrm{1}$$, and the two concentrations of the solid stuffs $$\ce{Zn}$$ and $$\ce{ZnO}$$ in their own phases. The mass of these stuffs may change but not their concentrations, as they are in the solid phase. So it is no use keeping such a formula where three parameters are constant. It is better to group all constant parameters on the same side of the = sign, and define a new constant like $$K_\mathrm{2}$$ : $$K_\mathrm{2} = K_\mathrm{1}\frac{[\ce{Zn}]}{[\ce{ZnO}]}= \frac{p\ce{(H2)}}{p\ce{(H2O)}}$$ With this formula and a numerical value for $$K_\mathrm{2}$$ you may calculate immediately how $$p\ce{(H2)}$$ changes when $$p\ce{(H2O)}$$ is varied. No need to know the concentration of the solids. This may be useful if you want to optimize the yield in producing $$\ce{H2}$$ gas through this reaction.
• I don't see your point. The ratio of the concentrations of Zn in metallic zinc and ZnO in the solid ZnO is a constant, which should not be important, and probably between $0.1$ and $10$. I don't speak of the amounts, in mass or in moles. I just speak about the concentrations. Nov 11, 2020 at 9:18