The minerals that fizz when hydrochloric acid is dropped on them are carbonates. Carbonates are compounds that contain the carbonate ion (CO32-). When hydrochloric acid (HCl) comes into contact with a carbonate, it reacts with the carbonate ion to form water (H2O), carbon dioxide gas (CO2), and a chloride salt of the metal that was present in the carbonate. The reaction can be represented as follows:

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

The presence of carbonates in a mineral can be easily identified by the effervescence (fizzing) that occurs when the mineral is exposed to hydrochloric acid. This reaction is commonly used in geology and mineralogy to identify carbonate-containing minerals. Some common carbonate minerals that fizz when treated with hydrochloric acid include calcite, dolomite, and magnesite.