Why do pure solids and liquids disappear from K expressions?
You will be able to: Omit pure condensed phases correctly while retaining aqueous and gaseous species.
Why do pure solids and liquids disappear from K expressions?
Adding another clean crystal to a saturated solution can leave its dissolved concentration unchanged. The amount of pure solid changes, but the solid has the same bulk composition and density.
A useful starting point: How do you turn a balanced reaction into Q and K? →
Words and symbols before equations
- Pure phase
- A substance present as its own pure solid or liquid phase.
- Heterogeneous equilibrium
- An equilibrium involving more than one phase.
- Aqueous, aq
- Dissolved in water; its concentration can vary.
- Activity
- Effective chemical participation; a pure condensed phase is assigned unity in this model.
What this picture assumes
CaCO₃(s) ⇌ CaO(s)+CO₂(g), both pure solids present at fixed T and V. Supplied equilibrium PCO₂=0.25 atm. Solid symbols are relative inventory, not an atom-scale or mass measurement.
Read the picture in three steps
- Read the species and labels first. Identify what each symbol and line represents. Read the units and fixed conditions before comparing quantities.
- Adding excess pure CaCO₃ changes solid amount, not the equilibrium CO₂ pressure, while both solid phases coexist at fixed temperature.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
In equilibrium expressions, a pure solid or pure liquid contributes a constant factor that is absorbed into K. Its amount is not inserted as a concentration variable.
For CaCO₃(s) ⇌ CaO(s)+CO₂(g), Kp=PCO₂ while both solids coexist at the specified temperature. Adding more of one pure solid does not change this equilibrium pressure.
Omission does not mean that solids are unimportant or absent. If a phase is completely consumed, the assumed coexistence equilibrium may no longer be possible.
An aqueous species is not a pure liquid. Keep its variable dissolved concentration. Water as a dilute-solution solvent is usually omitted under the stated approximation.
A worked example, step by step
For CaCO₃(s) ⇌ CaO(s)+CO₂(g), both solids coexist and PCO₂=0.25 atm at equilibrium. Find Kp and predict the effect of adding more CaCO₃(s) at fixed T and volume.
- Identify CO₂ as the only gaseous variable in the expression.
- Omit both pure solids: Kp=PCO₂.
- Kp=0.25 using the stated atm convention.
- More pure CaCO₃ does not change the equilibrium pressure while both phases remain and T is fixed.
Do not omit a substance merely because it contains water or has an aqueous label.
Does adding solid necessarily increase its equilibrium-expression concentration?
Compare with an explanation
No. The pure phase has constant activity in this model; more amount does not change that factor.
Predict. Change one thing. Explain.
Increase the displayed solid inventory while keeping temperature fixed. Compare the unchanged gas equilibrium pressure and note the phase-coexistence assumption.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Adding excess pure CaCO₃ changes solid amount, not the equilibrium CO₂ pressure, while both solid phases coexist at fixed temperature.
CaCO₃(s) ⇌ CaO(s)+CO₂(g), both pure solids present at fixed T and V. Supplied equilibrium PCO₂=0.25 atm. Solid symbols are relative inventory, not an atom-scale or mass measurement.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using relative rates, particle conservation, the Q/K comparison or the stated dissolution equilibrium. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionWrite Q for AgCl(s) ⇌ Ag⁺(aq)+Cl⁻(aq). Explain why each included or omitted term belongs, and why undissolved solid can still matter.
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Compare with the answer and four-point rubric
- 1 point: Q=[Ag⁺][Cl⁻].
- 1 point: Both aqueous concentrations can vary.
- 1 point: Pure AgCl(s) contributes a constant activity, so no solid concentration appears.
- 1 point: Remaining solid can dissolve or grow; if exhausted, saturation is not guaranteed.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1Is aqueous the same as pure liquid?
No; aqueous solute concentration is variable.
RECALL 2Does omission imply no physical role?
No; the phase supplies or receives matter.
RECALL 3What condition matters when changing solid mass?
Required pure phases must still coexist.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Why do pure solids and liquids disappear from K expressions?
- Omit pure solids and pure liquids from the usual AP Q/K expression.
- The phase must remain available for the stated coexistence equilibrium.
Remember: Do not omit a substance merely because it contains water or has an aqueous label.
Conditions: CaCO₃(s) ⇌ CaO(s)+CO₂(g), both pure solids present at fixed T and V. Supplied equilibrium PCO₂=0.25 atm. Solid symbols are relative inventory, not an atom-scale or mass measurement.
Refresh Kid · AP Chemistry Unit 7 · Objectives 7.3.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 7.3, objective 7.3.A. CED effective Fall 2024 and June 2026 clarifications checked September 17, 2026. Unit 7: Equilibrium, Topics 7.1–7.12. Focused lesson names, examples, models and assessments are original Refresh Kid teaching materials, not additional official topics or official AP questions. Official corrections.
The model states its assumptions beside the diagram. Converting between Kc and Kp and calculations for a dissolved species in equilibrium with its gas phase are excluded from assessed Unit 7 scope. Concentrations use mol/L and gas partial pressures use the stated pressure convention. Supplied constants are teaching data at fixed temperature unless otherwise specified. Ideal dilute-solution and ideal-gas approximations are stated. 3D views show inventories, not molecular trajectories, measured structures or proof of equilibrium from a single snapshot. Approximation checks are explicit; a small K alone does not justify neglecting every change.
Teaching resources: The Organic Chemistry Tutor video titles/descriptions and topic coverage were checked for optional links; no claim is made to have watched every video. No creator scripts, examples, worksheets or artwork were copied. GitHub’s 3D website collection and its Three.js camera-control example informed the idea of controllable spatial inspection. Scientific diagrams, geometry and interactions here are original. The self-hosted Three.js runtime retains its MIT license. Camera rotation changes the view, not the chemistry.
Independent teacher review and observation of students remain pending. Implementation checks do not certify scientific accuracy, accessibility or learning effectiveness. This is a review edition.
Optional official resource: Released AP Chemistry questions and scoring guides. This archive contains questions across units; it is not an assignment of every question to this lesson.
The teaching sequence is informed by the IES learning guide; this exact implementation has not been evaluated with learners.
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