This activity demonstrates the effect of pressure and temperature on three reversible reactions: the solubility of carbon dioxide in water, the reaction of dissolved carbon dioxide and water to form H2CO3, and the weak acid ionization of H2CO3 to give HCO3- and H+ ions. For simplicity sake in terms of classroom discussion, these reactions are combined into one equation in Equation 1. The position of equilibrium for this reaction can be determined by measuring the concentration of H+ ions in solution. Seltzer water is used as a source of dissolved carbon dioxide, and the concentration of H+ ions is estimated using
bromcresol green as an indicator. The indicator is yellow when the pH is less than 3.8, blue when the pH is greater than 5.2, and various shades of green in the pH range 3.8–5.2. A sealed syringe is used to provide a closed system.
2CO2(g) + H2O(l) → CO2(aq) + H+(aq) + HCO3 –(aq) Equation 1