Calcium carbonate is a common mineral that is found in many forms, including limestone, chalk, and marble. It is also found in some types of seashells and eggshells. When dilute hydrochloride acid (HCI) is placed on calcium carbonate, it fizzes and releases carbon dioxide gas. This is because the hydrochloric acid reacts with the calcium carbonate to produce calcium chloride and water, and the carbon dioxide gas is released as a byproduct of the reaction.