Measuring reaction rate. Image 1 of 2. Apparatus used to measure the rate of the reaction between calcium carbonate (CaCO3) and hydrochloric acid (HCl). The conical flask containing the reactants is on an electronic balance. During the reaction mass is lost, as some of the reactants are converted to gases. The cotton wool plug prevents liquid being ejected from the flask during the reaction, which fizzes quite violently. Measuring the change in mass over time allows the rate to be determined. The equation for the reaction is: CaCO3 + 2HCl ---> CaCl2 + H2O + CO2. See image A500/495 for the set-up at a more advanced stage.