When drops of hydrochloric acid (HCl) are dropped on marble (calcium carbonate, CaCO3), carbon dioxide gas (CO2) bubbles out. This reaction can be represented by the following chemical equation:

2HCl + CaCO3 → CaCl2 + H2O + CO2

In this reaction, the hydrochloric acid reacts with the calcium carbonate to form calcium chloride (CaCl2), water (H2O), and carbon dioxide gas (CO2). The carbon dioxide gas produced is what causes the bubbles to form.