# Kc A level.

i don't understand why the conc of solids and liquids are essentially constant, and i dont understand why that means they're omitted from the Kc expression. I also don't understand how they're automatically incorporated within the overall equilibrium constant - just someone please helppp
(edited 1 month ago)
It's no wonder that you are confused, because at A Level the concept of equilibrium constant is oversimplified and it is often not made clear that the Kc you are calculating using concentrations (or partial pressures for gases) is just an approximation. The actual thermodynamic Kc is calculated from the ratios of the activities of the components, where the activity is a dimensionless number related to the effective concentration relative to some standard state. For dilute solutions, it is usually a good approximation to replace activity by the concentration of solution relative to the standard concentration of 1 M. Similarly, for gases, the activity can be approximated by the partial pressure relative to standard pressure of 1 atm.
However, for pure solids or liquids, the activity is defined to be 1 so there is no need to write any term in Kc for them. It's not really because they have 'constant' concentration, but rather that their activity is 1 that they can be omitted from the expression for Kc. Of course, if different standard states (other than 1M or 1 atm) are chosen then the activities will be different from 1, but that would have no effect on the shift in concentrations/pressure of the liquid/gas components of equilibrium as the amounts are varied. For further reading, I recommend Atkins Physical Chemistry Chapter 7, although this goes well beyond A level syllabus.
Just to add to the (excellent) post above.

The law of mass action refers to the "activity" of the particles, which for pre-university students approximates to the concentration.

BUT in solids the particles are only "active" at the surface, which is in contact with the other reagent(s), and even there, the particles have only one direction from which to collide. Most of the particles in a solid are not available for reaction (they are on the inside). Hence, the particles in a solid do not have an "activity" as such, and so to solve the problem they are assumed to not interfere in the law of mass action (equilibrium).

The same argument can be applied to non-homogeneous liquids that have a phase interface.