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Write Kp for CaCO₃(s) ⇌ CaO(s) + CO₂(g) and most students get p(CO₂) without thinking. Ask them why the solids vanish, or what happens when you add more solid, or when the solid runs out, and the answers get shaky. This post explains the reason in one line, then works three problems on ammonium hydrosulphide with numbers you can check.
What does NCERT say?
Section 6.5 of NCERT's Class 11 unit Equilibrium (2026-27 reprint) defines it: "Equilibrium in a system having more than one phase is called heterogeneous equilibrium." The key sentence follows: the expressions simplify "as the molar concentration of a pure solid or liquid is constant (i.e., independent of the amount present)". Then a condition many students miss: "pure solids or liquids must also be present (however small the amount may be) at equilibrium, but their concentrations or partial pressures do not appear in the expression".
The JEE (Main) 2026 syllabus lists solid-liquid, liquid-gas, gas-gas and solid-gas equilibria in Unit 6, with Kp and Kc. The NEET (UG) 2026 syllabus lists solid-liquid, liquid-gas and solid-gas equilibria and the same constants.
Why is a pure solid's concentration constant?
Concentration is moles per litre of the phase itself. Double the lump of solid and you double both its moles and its volume, so the ratio stays fixed at a given temperature. Liquid water is a good picture: about 1000 g ÷ 18.02 g/mol ≈ 55.5 mol in every litre, whether the beaker holds a litre or a drop. Since that number cannot change, it is folded into K.
Two consequences follow. Adding more solid does not shift the equilibrium. And for Kp = Kc(RT)^Δn, Δn counts gas moles only.
Worked example 1: the dissociation pressure
Sample numbers. Pure NH₄HS is placed in an evacuated flask:
NH₄HS(s) ⇌ NH₃(g) + H₂S(g)
At equilibrium the total pressure is 0.60 bar. Both gases come from the solid in a 1 : 1 ratio, so each has 0.30 bar, and
Kp = p(NH₃) × p(H₂S) = 0.30 × 0.30 = 0.090 bar².
The solid does not appear. For converting to Kc, Δn = 2 − 0 = 2; the solid is not a reactant in the gas count.
Worked example 2: a common gas already present
Same temperature. This time the flask already holds NH₃ at 0.20 bar before the solid goes in. Let x be the pressure of H₂S formed.
p(NH₃) = 0.20 + x, p(H₂S) = x.
(0.20 + x)x = 0.090, so x² + 0.20x − 0.090 = 0.
x = [−0.20 + √(0.04 + 0.36)] ÷ 2 = (−0.20 + 0.632) ÷ 2 = 0.216 bar.
So p(H₂S) = 0.216 bar, p(NH₃) = 0.416 bar, total 0.632 bar.
Check: 0.416 × 0.216 = 0.0899 ≈ 0.090. Less H₂S forms than in example 1 (0.216 against 0.30 bar), as Le Chatelier's principle predicts for an added product.
Worked example 3: when the solid runs out
Sample numbers again. Suppose Kp = 0.090 bar² applies at 300 K, and you put only 0.0050 mol of NH₄HS into an evacuated 10.0 L flask. Use R = 0.0831 bar L mol⁻¹ K⁻¹, as NCERT does.
To reach equilibrium, each gas needs 0.30 bar, which is n = pV/RT = (0.30 × 10.0) ÷ (0.0831 × 300) = 0.120 mol of each.
You only have 0.0050 mol of solid. All of it decomposes, giving 0.0050 mol of each gas.
Each gas is then at (0.0050 × 0.0831 × 300) ÷ 10.0 = 0.0125 bar.
Q = 0.0125 × 0.0125 = 1.6 × 10⁻⁴ bar², far below K.
No solid is left, so there is no equilibrium, exactly as NCERT's "must also be present" condition warns. Questions that give you a flask volume and a small amount of solid are testing this.
What about liquids and solutions?
Vapour pressure. For H₂O(l) ⇌ H₂O(g), K = p(H₂O). That is why vapour pressure depends only on temperature, not on how much liquid is in the container.
Saturated solutions. NCERT's example is Ca(OH)₂ with its saturated solution. The ions stay in the expression and the solid drops out, which is the basis of the solubility product.
Solvent water. In NCERT's Ag₂O + HNO₃ example, water formed as H₂O(l) is left out too.
Which slips cost marks?
Writing the solid in the K expression.
Counting the solid in Δn.
Claiming that adding more solid shifts the equilibrium.
Forgetting that the solid can run out.
Dropping aqueous species along with the solid. Only pure solids and pure liquids leave.
If the idea of dynamic equilibrium itself is unclear, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, each with its own lesson.
A practice routine
Write Kp and Kc, with Δn, for five heterogeneous reactions, including Ni(s) + 4CO(g) ⇌ Ni(CO)₄(g) from NCERT.
Redo example 2 with H₂S at 0.10 bar already present.
Redo example 3 and find the smallest amount of solid that would let equilibrium be reached.
Then work NCERT Problem 6.6 on CO₂ and graphite.
For the rest of the chapter's traps, use our checking routine for equilibrium answers. Start with step 1 tonight, writing the phase label on every species.
