The chemical formula for calcium carbonate, which is the active ingredient in Tums, is "CaCO3," according to GlaxoSmithKline. The active ingredient in a product is the ingredient that causes the intended effect.
Heartburn is a symptom of the stomach producing too much hydrochloric acid, which is one of the substances found in the gastric juice that helps break down food in the stomach. Commonly, over-the-counter products such as Tums contain weak bases as their active ingredients in order to neutralize the acid in the gastric juice. Over-the-counter antacids are an effective way to treat heartburn because they treat the chemical cause.