Q.Explain why pure liquids and solids can be ignored while writing the equilibrium constant expression?

The key idea is that the activity of a pure solid or liquid is taken as unity (1) because its concentration does not change during the reaction — only its surface area matters, and that is constant.

Reasoning:

1. The equilibrium constant $K$ is defined in terms of activities (effective concentrations). For a pure solid or liquid, the activity is $a = 1$ at a given temperature and pressure.
2. In the reaction quotient $Q$, the term for a pure substance is $1$ — it does not vary with amount, so it contributes nothing to the ratio. …

The core idea: why "constant" matters

Equilibrium constant expressions are built from activities (effective concentrations), not raw amounts. For a pure solid or pure liquid, the activity is defined as 1 — not because it's absent, but because its concentration doesn't change as the reaction proceeds.

Think about it: if you have a lump of solid calcium carbonate in a flask, its "concentration" (moles per litre of the solid itself) is fixed by its density and molar mass. Adding more solid doesn't make it more concentrated — it just gives you a bigger lump. The same holds for a pure liquid like water in a dilute solution: its concentration stays roughly 55.5 M regardless of how much reaction happens.

Since the equilibrium constant $K$ is a ratio of activities, and the activity of a pure solid or liquid is always 1, that factor simply multiplies the numerator or denominator by 1 — so we drop it.

Step-by-step reasoning

1. Define the equilibrium constant properly The thermodynamic equilibrium constant $K$ is defined using activities ($a$), not molar concentrations. For a reaction

the expression is

2. What is the activity of a pure substance?

   For an ideal gas, activity ≈ partial pressure. For a solute in dilute solution, activity ≈ molar concentration. But for a pure solid or pure liquid, the activity is taken as 1 (by convention, at standard state).

   Important

   The standard state of a pure solid or liquid is the substance itself at 1 bar pressure and the temperature of interest. Its activity is defined as exactly 1.

3. Why is activity = 1 physically reasonable?

   The "concentration" of a pure solid is $\frac{\text{density}}{\text{molar mass}}$, which is a fixed number at a given temperature. For example, solid iron has density 7.87 g/cm³ and molar mass 55.85 g/mol, giving a concentration of about 141 mol/L. This number doesn't change whether you have 1 g or 1 kg of iron — it's an intensive property. So the activity remains constant, and we set it to 1 for convenience.

4. See it in action with a real reaction

   Consider the thermal decomposition of calcium carbonate:

The full equilibrium expression would be:

Since $a_{\ce{CaO}} = 1$ and $a_{\ce{CaCO3}} = 1$, this simplifies to: