Practice question A reaction occurs in an open flask: CaCO₃(s) + 2 HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). The mass of the flask and contents is 86.35 g just after mixing and 85.47 g when the reaction finishes. Calculate the decrease and explain, using the particle model, why the measured mass changes despite conservation of mass.