Answer:
You can explain it to your roommate like this:
"Antacids work by neutralizing the excess acid in your stomach that is causing the heartburn. The active ingredient in the antacids, such as calcium carbonate, reacts with the stomach acid to form a neutral substance, which helps to relieve the symptoms of heartburn. The antacids also help to increase the pH level in your stomach, which reduces the production of stomach acid and can provide quick relief from heartburn."