NCEA Level 3 Chemistry (AS91393) Practice Exam 2026 – Complete Aqueous Chemistry Prep

When carbon dioxide dissolves in water, it forms carbonic acid (H₂CO₃) through the following reaction:

\[ \text{CO₂ (g) + H₂O (l) ⇌ H₂CO₃ (aq)} \]

This carbonic acid can then dissociate into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺):

\[ \text{H₂CO₃ (aq) ⇌ H⁺ (aq) + HCO₃⁻ (aq)} \]

The release of hydrogen ions into the solution increases the concentration of H⁺ ions, which leads to a decrease in pH. A lower pH indicates a more acidic solution. Therefore, the correct answer indicates that dissolving carbon dioxide in water results in an acidic environment due to the formation of carbonic acid and the subsequent release of hydrogen ions.

The other potential options, such as increased pH or unchanged pH, do not accurately reflect the chemical behavior of CO₂ in water. The fluctuation of pH with temperature changes may occur in various reactions, but in this case, the primary effect of dissolved CO₂ is a measurable decrease in pH due to acid formation.