When an equilibrium mixture is disturbed without changing temperature, the equilibrium constant $K$ remains fixed. The mixture responds because the disturbance changes the reaction quotient $Q$.

Consider

$$\mathrm{A+B\rightleftharpoons C}$$

with

$$K_c=\frac{[\mathrm C]}{[\mathrm A][\mathrm B]}.$$

If additional A is injected into a mixture that was initially at equilibrium, $[\mathrm A]$ increases immediately while the other concentrations have not yet reacted. The denominator of $Q_c$ increases, so

$$Q_c<K_c.$$

Net reaction therefore proceeds forward until $Q_c$ again equals $K_c$. This is commonly described by saying that the equilibrium shifts toward products.

Likewise:

• adding a product tends to make $Q>K$ and drives net reaction toward reactants;
• removing a reactant tends to make $Q>K$ and drives net reaction toward reactants;
• removing a product tends to make $Q<K$ and drives net reaction toward products.

These tendencies are summarized by Le Châtelier's principle: when an equilibrium system is perturbed, its composition changes in a direction that partially opposes the perturbation. Comparing $Q$ with $K$ gives the quantitative reason for the direction.

Changing the volume of a solution can change all molar concentrations at once. The outcome depends on the equilibrium expression rather than on a universal rule. For

$$\mathrm{A\rightleftharpoons B},$$

uniform dilution changes both $[A]$ and $[B]$ by the same factor, so the ratio

$$Q_c=\frac{[B]}{[A]}$$

is unchanged and no shift is predicted. For reactions with different total powers of concentration in numerator and denominator, dilution can change $Q$ and produce a shift.

Temperature is fundamentally different from merely adding or removing material: changing temperature can change the value of $K$ itself. Predicting that dependence requires the thermochemistry of the reaction and is treated separately.