When a dilute acid, such as hydrochloric acid (HCl), is added to a sample of limestone (calcium carbonate, CaCO3), the rock fizzes due to the release of carbon dioxide gas (CO2). This reaction can be represented by the following chemical equation:

CaCO3 (limestone) + 2HCl (hydrochloric acid) → CaCl2 (calcium chloride) + H2O (water) + CO2 (carbon dioxide gas)

The presence of carbon dioxide gas bubbles indicates that the rock contains calcium carbonate, which is a common component of limestone. This fizzing reaction is a simple and quick way to test for the presence of carbonate minerals in a rock sample.