An antacid is any substance, generally a base or basic salt, which counteracts stomach acidity. In other words, antacids are stomach acid neutralizers. Most all antacids are created with one of the following chemicals:

* Aluminium hydroxide (Amphojel, AlternaGEL) * Magnesium hydroxide (Phillips' Milk of Magnesia) * Aluminum hydroxide with magnesium hydroxide (Maalox, Mylanta, Diovol) * Aluminum carbonate gel (Basaljel) * Calcium carbonate (Alcalak, TUMS, Quick-Eze, Rennie, Titralac, Rolaids) * Sodium bicarbonate (Bicarbonate of soda, Alka-Seltzer) * Hydrotalcite (Talcid) * Bismuth subsalicylate (Pepto-Bismol) * Magaldrate with Simethicone (Pepsil) All these chemicals happen to be mostly insoluble in water, making an inherent property of antacids being insoluble.