The above is a simplification of a multi-step equilibrium between CO₂ (g) ⇌ CO₂ (aq) + H₂O (l) ⇌ H₂CO₃ (aq) ⇌ H ⁺ (aq) + HCO₃ ^- (aq) ⇌ H ⁺ (aq) + CO₃ ^2- (aq) . For our purposes the last two steps of this equilibrium are most important to understand the behaviour of the solution. In particular, when we change the amount of CO₂ (g) it affects the state of the equilibrium. This determines the concentration of hydrogen ions (H ⁺ (aq)) in the solution and thus the pH.

The pH indicator we used (methyl red) behaves according to HIndicator ⇌ H ⁺ + Indicator ^- . It turns red in acidic solutions and yellow in basic solutions. By removing CO₂ (g) in the trial, the equilibrium shifts to the left. Because this reduces the concentration of H ⁺ (aq), the solution is more basic, which explains the colour change to yellow.