Heartburn is characterized by a burning sensation in the chest behind the breastbone. It occurs when stomach acid moves up into the esophagus. When you swallow, a band of muscle called the lower esophageal sphincter loosens to give way for food and liquid to enter the stomach, after which it tightens once more. But when the lower esophageal sphincter slackens abnormally, stomach acid may flow up, a phenomenon called acid reflux occurs, which results in heartburn. Antacids are usually associated with heartburn first aid, but there are other ways to alleviate this condition.