The major component of eggshells is calcium carbonate, CaCO3(s). This analysis is done volumetrically by using a characteristic reaction of carbonate compounds, namely their reaction with acids. Calcium carbonate (limestone) is very insoluble in pure water but readily reacts in acid according to the reaction below. 2HCl(aq) + CaCO3(s) → CaCl2(aq) + CO2(g) + H2O(l) + HCl(aq) (1) (in excess) (limiting reagent) (unreacted) This reaction cannot be used directly to titrate the CaCO3 because it is very slow when the reaction is close to the endpoint. Instead the determination is achieved by adding an excess of hydrochloric acid to react with all of the CaCO3 and then titrating the remaining unreacted HCl with NaOH solution to determine the amount of acid which did not react with the calcium carbonate. The difference between the moles of the acid (HCl) initially added and the moles of HCl left unreacted after the reaction, is equal to the moles of HCl that did react with CaCO3. The reaction used to determine the amount of unreacted acid by titration is given below. This type of analysis is generally referred to as a back-titration. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) (2) (unreacted)