For example, sodium hydroxide (NaOH) is a base. It falls apart in water, as acids do, but it splits into a positively charged sodium ion (Na⁺)and a negatively charged hydroxide ion (HO^-). The hydroxide ion can steal a hydrogen ion from an acid molecule like acetylsalicylic acid.

Look at that! When the hydroxide ion took the hydrogen ion, the two joined to form a water molecule.

So now we're left with a water molecule, an acetylsalicylate ion, and a sodium ion. The acetylsalicylate ion can combine with the sodium ion to form a molecule of sodium acetylsalicylate. This molecule is called a salt. A salt is the product (other than water) that is made when an acid and base react. A salt you know well is made when sodium hydroxide reacts with hydrochloric acid.

NaCl is just plain old table salt.

A sodium salicylate   remedy bottle.

Salt chemistry gives us a footnote in the story of aspirin. Remember that before aspirin was developed, people used salicylic acid (SA) as a pain reliever. But SA made people's stomachs hurt. Some people thought that this was because SA is an acid. So they neutralized salicylic acid with sodium hydroxide to make sodium salicylate, which is a salt. Sodium salicylate was used as a pain reliever, but what do you know, it caused just as much stomach pain as SA! So the pain obviously was the result of something other than SA's acidity.

Today we know that SA is a really weak acid, and since your stomach is already filled with lots of hydrochloric acid (which is a strong acid), SA's acidity really didn't affect your stomach that much. We also have learned that SA and aspirin interfere with a compound called COX-1 that helps your body protect the stomach, and this interference causes the stomach aches. However, aspirin causes far fewer stomach aches than SA!

A sodium salicylate remedy bottle: Courtesy Marvin Samson Center for the History of Pharmacy, University of the Sciences in Philadelphia.