What does it mean that acids donate hydrogen ions while bases accept them?

Answer

It means that when you add an acid to water, for instance hydrochloric acid, or HCl, it will break apart into two ions H⁺ and Cl^-. The H⁺ is the hydrogen ion. The H⁺ released is the "active" ingredient in an acid.

On the other hand, when you add a base to water, it doesn't release H⁺, but rather it takes H⁺ that is already in the solution and bonds to it. The most common way this happens is with the hydroxide ion, or OH^-. For instance, if you have the base potassium hydroxide, KOH, and you add it to water, it will form the ions K⁺ and OH^-. The OH^- will very quickly react with any H⁺ in the solution and when that happens, you form water: H⁺ + OH^- --> H₂O

So acids give off H⁺ into the water, and bases remove it. A simple analogy is a sponge. Acids are like wet sponges -- when you squeeze it the liquid comes out. Bases are like dry sponges -- when you put water on it, it soaks it up.

Note: there are other more complete definitions of acids and bases that don't involve H⁺ at all. In fact, it is more accurate to say that an base is a species that donates electron pairs, and an acid is a species that accepts electron pairs. This definition, while much more complete, is somewhat harder to explain on a simple level however.

First answer by Afaqi. Last edit by JEK. Contributor trust: 1862 [recommend contributor]. Question popularity: 63 [recommend question]