Watching a reaction happen
When a chemical reaction takes place, the reactants are used up and new products are formed. So when we watch a reaction, there are two things we could be watching.
calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
On the left — the reactants
The limestone and the acid. As time passes there is less of each of them.
On the right — the products
Calcium chloride, water and carbon dioxide. As time passes there is more of each of them.
Which one fixes the rate?
The rate of the reaction can be settled either from how fast a reactant is used up or from how fast a product is formed. Both describe the same reaction, and they are tied to one another — if the limestone disappears quickly, the carbon dioxide appears quickly too.
The easiest thing to watch
In this reaction the easiest things to observe are the rate at which the limestone is used up and the rate at which carbon dioxide is given off. The first can be weighed, and the second can be seen as bubbles.