Heartburn is a painful burning sensation in the esophagus (the tube that joins the throat to the stomach ) just below the chest. The pain is caused by hydrochloric acid (HCl), which is excreted in the stomach to kill microorganisms and activate enzymes that breakdown food. the hydrochloric acid can sometimes back up out of the stomach and into the esophagus, a phenomenon called acid reflux. Recall from Section 4.8 that acids are substances that by one definition that we elaborate on shortly-produce H ions in solution. When hydrochloric acid from the stomach comes in contact with the lining of the esophagus, the H ions irritate the esophageal tissues, resulting in the burning sensation, some of the acid can work its way into the lower throat and even the mouth, producing pain in the throat and as our teste [characteristic of acids] in the mouth. Almost everyone experiences heartburn at sometime, most commonly after a large meal when the stomach is very full and the chances for reflux are greatest. Strenuous activity or lying in a horizontal position after a large meal increases the likelihood of stomach acid reflux and the resulting heartburn.