Why do we need antacids?

Water dilutes acid. If you have excess stomach acid, why not just drink a quart of water to dilute and rinse out the acid?

I would imagine that whatever mechanisms normally regulate acid production would sense the increased pH and pump out more acid to compensate. While some antacids like Tums are just calcium carbonate and neutralize some of the stomach acid, other drugs like omeprazole temporarily inhibit acid production to provide relief.

Also, if part of your issue is overeating, a quart of water is only going to exacerbate the situation but a teeny alka-seltzer won’t take up that much space.

I think you’re fundamentally misunderstanding the problem of heartburn. It isn’t really too much acid. It’s that stomach acid has gotten out of the stomach and up into the esophagus. The esophagus is sensitive to this (lacking the stomach’s protective lining) and so it starts to hurt.

One reason for acid to get out is that the stomach is very full. If you think about adding water in this context, you’ll see that it’s not a solution - while you may dilute the acid, you’ve only added more volume to the stomach contents and made it that much harder to prevent acid from getting to the esophagus.

A second problem with the water idea is this: where is the water going? The water may dilute the acid for a while, but your body is selectively absorbing water into the bloodstream, making sure that acid stays in the digestive tract. Thus, the water can’t “rinse out” the acid to anywhere. (Any situation that involved rinsing out would translate to really, really bad diarrhea.)

Think of it as throwing water on an oil fire. Water puts out fire, right? But all you’re going to do is make the fire jump out of the pan (up the esophagus) along with the oil. Much better to coat it with a layer of something that puts out fires and/or absorbs oil.

But heartburn is only one of a number of syndromes due to excess acid. You left out peptic ulcer disease (i.e. stomach and duodenal ulcers due in large part to acid).

In any case, while drinking a quart of water would dilute the acid, it would soon pass down and past the stomach and duodenum soon leaving the person back with the original level of acid. You’d have to drink water almost non-stop (and, assuming your kidneys are normal, you’d be peeing non-stop too). Besides, water’s not even that great an acid neutralizer (although, OTOH, milk is).

I didn’t know the body could be quite that selective. The acid would be in solution in the water–how would the digestive tract separate it out and absorb only the water?

I wasn’t thinking of rinsing it through the entire system, just out the stomach, where I thought that most of the issues about acid occur.

Which almost literally, is what Gaviscon does.