Final answer:
The completion of the reaction between HCl and carbonate can be determined by observing when gas bubbles cease to form.
Step-by-step explanation:
The completion of a reaction between HCl and carbonate can be determined by observing certain changes. One indication is the ceasing of gas bubbles. When HCl reacts with carbonate, it produces carbon dioxide gas, so when the gas bubbles stop forming, it suggests that the reaction is complete.