Equilibrium expressions are powerful only when written correctly. The most common errors come from including the wrong species, especially pure solids and pure liquids, or mixing up $K_c$ and $K_p$.

What to omit from $Q$ and $K$

The core rule (heterogeneous equilibria)

In any equilibrium expression (both reaction quotient $Q$ and equilibrium constant $K$), omit:

• Pure solids (e.g., $\mathrm{CaCO_3(s)}$, $\mathrm{Fe(s)}$)

• Pure liquids (e.g., $\mathrm{H_2O(l)}$, $\mathrm{Br_2(l)}$)

Include species only when their amounts can change the “effective concentration” in a meaningful way, such as:

This graph shows that the solubility (concentration) of several gases in water changes substantially with temperature even at constant pressure. It visually reinforces why gaseous species are treated as composition-dependent terms in equilibrium expressions, unlike pure condensed phases whose activities are taken as approximately 1. Source

• Aqueous solutes (use bracket concentration, e.g., $[\mathrm{Na^+}]$)

• Gases (use partial pressures for $Q_p/K_p$)

Why pure solids and pure liquids are omitted

Their “concentrations” do not meaningfully change as long as some of the phase is present:

• A pure solid has essentially constant density, so its molar amount per unit volume of solid is constant.

• A pure liquid similarly has nearly constant density (and thus constant molar concentration for the pure liquid).

These constant factors are mathematically absorbed into the constant itself, so the equilibrium expression is written only in terms of variable composition species (gases and dissolved solutes).

Activity viewpoint (what the rule is really saying)

Because the activity of a pure solid or liquid is ~1, including it would multiply the expression by ~1 and change nothing, so it is omitted by convention.

How to write expressions without the omitted phases

Write $Q$ or $K$ using only species whose measured amounts can vary in the mixture (typically aqueous and gaseous species), with exponents equal to stoichiometric coefficients.

This same “include/omit” logic applies identically to $Q_c$ and $Q_p$.

Common traps involving $\mathrm{H_2O}$

Water is frequently present, so state symbols matter:

• Omit $\mathrm{H_2O(l)}$ when it is a pure liquid (even if it appears as a reactant or product in the balanced equation).

• Include $\mathrm{H_2O(g)}$ because it is a gas (its partial pressure can change).

• Include $\mathrm{H_2O(aq)}$ only if it is being treated as a solute in a nonstandard way (rare in AP-level contexts; most often you won’t see water written as $\mathrm{(aq)}$).

A related idea: in reactions occurring in water, the solvent is not written into $K$ as a “reactant” because its effective concentration is treated as constant under the conditions assumed.

“Be aware of whether $K_c$ or $K_p$ is used” (AP exam note)

Always match the equilibrium constant form to the data provided:

These two expressions highlight the structural parallel between $K_c$ and $K_p$: both are products-over-reactants with exponents equal to stoichiometric coefficients. The key difference is the measurable quantity used—molar concentrations for $K_c$ versus partial pressures for $K_p$. Source

• If the problem gives molar concentrations or refers to species in solution, it is typically $Q_c/K_c$.

• If the problem gives partial pressures for gases, it is $Q_p/K_p$.

Use consistent quantities within one expression (don’t mix $[\ ]$ and $P$ in the same $Q$ or $K$ expression unless a problem explicitly defines a nonstandard approach).

Conversion note (what you are not required to do)

You should recognise that $K_c$ and $K_p$ are different forms, but converting between $K_c$ and $K_p$ is not assessed on the AP Exam. Your task is primarily to choose the correct form and write the expression correctly, omitting pure solids and pure liquids.