The best statement that would actually best describe this would be "caco3 is a reactant; it is present before the reaction occurs.". We can see this as noted in the first part of this equation, "CaCO3 + 2HCl →" would show that they would be positive, and also the this would be a reactant, and also for the second part of the equation, we also see how "aCl2 + CO2 + H2O" would always be present before the reaction would then occur.
This would therefore allow us to understand this equation clearly and to elaborate an to get the point of what this would really mean.
