Answer: The indicator that indicates the reaction has occurred is the formation of gas.
Explanation:
The common indicators of a chemical change has occurred are:
- Release of gas from the reaction.
- Formation of bubbles.
- Color change
- Temperature change
- Formation of a precipitate
- Specific odor
The chemical equation for the reaction of calcium carbonate and hydrochloric acid follows:
\(CaCO_3(s)+2HCl(aq.)\rightarrow CaCl_2(s)+H_2O(l)+CO_2(g)\)
As, we can see, gas (carbon dioxide) is being released during the reaction with effervescence.
Hence, the indicator that indicates the reaction has occurred is the formation of gas.
