science Q

Really depends on what kind of acid. Beverages like coke or fruit juices both contain acids. For coke, it's carbonic acid, and for citrus fruits, it's citric acid (and about a million others (exaggeration)).
If a person were to down HCl or sulfuric acid or any other strong acid though...that's a whole different story. Proteins are almost instantly denatured, breaking down cellular structures and causing necrosis. Rapid dehydration occurs, causing cells to lose water rapidly. And when cells are damaged this much, apoptosis occurs; cells that lose water detach from one another as well. This leads to ulcers forming in one's alimentary canal (throat, esophagus, etc). It is excruciating.