Eggshells contain a high percentage by mass of calcium carbonate, CaCO₃. A student wants to find out what percentage of an eggshell is calcium carbonate and uses the following method. This method uses a known excess of acid to dissolve the eggshell. The amount of unreacted acid is then determined by titration with an alkali. Assume the acid only reacts with the CaCO₃ in the eggshell. step 1 Wash an empty eggshell with distilled water. step 2 Warm the eggshell in an oven for a few minutes until dry. step 3 Grind the eggshell into a powder. step 4 Weigh approximately 2 g of the eggshell powder into a conical flask using a balance which measures to three decimal places. step 5 Add 100 cm³ of 2.00 mol dm⁻³ hydrochloric acid to the conical flask. step 6 Loosely cover the conical flask and leave for two days. step 7 Filter the contents of the conical flask, with any rinsings, into a 250.0 cm³ volumetric flask and top-up to the mark using distilled water. step 8 Transfer 25.00 cm³ of the solution prepared in step 7 into a conical flask, add a few drops of thymol blue indicator and titrate against 1.00 mol dm⁻³ sodium hydroxide using a 50 cm³ burette. The calcium carbonate in the eggshell reacts with the excess hydrochloric acid as follows. CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l) The excess acid reacts with the sodium hydroxide solution as follows. NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)