How does CO2 dissolve in seawater and what is the effect on ocean pH?

• A. CO2 reacts with water to form carbonic acid, lowering ocean pH and causing acidification.
• B. CO2 dissolves without reacting; pH increases.
• C. CO2 forms bicarbonate which increases pH.
• D. CO2 dissolution has no effect on pH.

Answer: (A)

CO2 dissolving in seawater engages the carbonate chemistry of the ocean. When CO2 enters, it reacts with water to form carbonic acid (CO2 + H2O ⇌ H2CO3). This carbonic acid then dissociates, releasing hydrogen ions and forming bicarbonate (H2CO3 ⇌ H+ + HCO3−), with further dissociation to carbonate (HCO3− ⇌ H+ + CO3^2−). The additional hydrogen ions raise the acidity of seawater, lowering its pH. While some bicarbonate and carbonate are produced, the overall effect of adding CO2 is an increase in H+ and a decrease in pH—ocean acidification. The other statements ignore or misstate this chemistry: CO2 does not dissolve without reacting and increasing pH, nor is there no pH effect.