The experiment you described is commonly used to test for the presence of carbon dioxide gas. When hydrochloric acid (HCl) is added to calcium carbonate (CaCO3), it reacts to form calcium chloride (CaCl2), water (H2O), and carbon dioxide (CO2) gas. The chemical equation for this reaction is:

HCl + CaCO3 → CaCl2 + H2O + CO2

The carbon dioxide gas produced in this reaction can then be detected by bubbling it through limewater (a saturated solution of calcium hydroxide, Ca(OH)2). If carbon dioxide gas is present, it will react with the calcium hydroxide to form calcium carbonate, which appears as a milky white precipitate. The chemical equation for this reaction is:

Ca(OH)2 + CO2 → CaCO3 + H2O

Therefore, by adding hydrochloric acid to calcium carbonate and transferring the gas to limewater, you can test for the presence of carbon dioxide gas.