This drawing represents a comet bringing atmospheric molecules and possibly primitive life forms to the Earth's surface. Click on image for full size (68K jpeg) Image courtesy of JPL/NASA

An acid is a substance which donates a proton to another species in a reaction. (Donate is really a passive term for what happens. The acid "forces" a proton on a substance which has any capability of accepting it. An acid "donates" a proton the way card players "donate" the Old Maid). Acids are classified as either strong or weak depending upon their relative ability to do this. When it does this, it changes the substance it attacks into a different substance.

There are many acids. Here are a few: Acetic acid (found in viengar), Acetylsalicyclic acid (found in aspirin), Ascorbic acid (found in vitamin C), Citric acid (found in lemon juice), Hydrochloric acid (digestive fluid in the stomach), Sulfuric acid (battery acid). In Earth science however, there only three which really matter because they contribute to weathering of terrestrial rocks. Those are Hydrochloric acid, Sulfuric acid, and Carbonic acid (which plays a role in the Earth's carbon cycle).