Antacids are a class of medicines that neutralize acid in the stomach. They contain ingredients such as aluminum, calcium, magnesium, or sodium bicarbonate which act as bases (alkalis) to counteract stomach acid and make its pH more neutral.
pH is a measure of the concentration of hydrogen ions in a solution and this determines how acidic or how alkaline that solution is. The scale ranges from 1 to 14, where below 7 is acidic, 7 is neutral, and above 7 is alkaline. Normal gastric acid pH is in the range 1.5-3.5.