hydrogen
The strength of an acid depends on how many hydrogen ions it produces when it dissolves in water. A stronger acid produces more hydrogen ions than a weaker acid. For example, sulfuric acid H 2 SO 4 , which is found in car batteries, is a strong acid because nearly all of it breaks down into ions when it dissolves in water. On the other hand, acetic acid CH 3 CO 2 H , which is the acid in vinegar, is a weak acid because less than 1 percent of it breaks down into ions in water.