The slow reaction can be shown by addition of a saturated aqueous solution of CO2, on the one hand, and of dilute acetic acid, on the to solutions of dilute NaOH containing phenolphthalein Indicator. The acetic acid neutralization instantaneous whereas with the CO2 neutralization it takes several seconds for the color to fade.
The hydration of CO2 occurs by two paths. For pH < 8 the principal mechanism is direct hydration of CO2: