To most geologists, the term “acid test” means placing a drop of dilute (5% to 10%) hydrochloric acid on a rock or mineral and watching for bubbles of carbon dioxide gas to be released. The bubbles signal the presence of carbonate minerals such as calcite, dolomite, or one of the minerals listed in Table 1.

The bubbling release of carbon dioxide gas can be so weak that you need a hand lens to observe single bubbles slowly growing in the drop of hydrochloric acid – or so vigorous that a flash of effervescence is produced. These variations in effervescence vigor are a result of the type of carbonate minerals present, the amount of carbonate present, the particle size of the carbonate, and the temperature of the acid.