What must be the nature of the substance that make up antacids?

They must neutralize stomach acids

Antacids are the substances that are used to control or neutralise the stomach acid. Antacids contains calcium carbonate, sodium bicarbonate and magnesium hydroxide. These substances have the ability to neutralise the gastric acid and thus removes the discomfort caused by acidity. So for the substances that make up antacids, must be able to neutralise acid.