The formation of bubbles is a clear indication that a reaction has occurred between calcium carbonate and hydrochloric acid. This is because the reaction between these two substances produces carbon dioxide gas, which causes the formation of bubbles.

The chemical equation for the reaction between calcium carbonate and hydrochloric acid is:

CaCO3 + 2HCl → CaCl2 + CO2 + H2O

In this reaction, calcium carbonate reacts with hydrochloric acid to form calcium chloride, carbon dioxide gas, and water. The carbon dioxide gas produced in this reaction is what causes the formation of bubbles.

The formation of bubbles is a visible sign that a reaction has occurred, and it can be used to confirm that the reaction between calcium carbonate and hydrochloric acid has taken place.