Carbon dioxide reacts with seawater and forms carbonic acid (H2CO3). The reaction can be represented by this equation: CO2 + H2O --> H2CO3 H2CO3 is a weak acid which can decrease the pH of seawater by releasing H+ H2CO3 <---> HCO3- + H+ HCO3- <--- > CO3^2- + H+ Solution Carbon dioxide reacts with seawater and forms carbonic acid (H2CO3). The reaction can be represented by this equation: CO2 + H2O --> H2CO3 H2CO3 is a weak acid which can decrease the pH of seawater by releasing H+ H2CO3 <---> HCO3- + H+ HCO3- <--- > CO3^2- + H+.