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Name: Jeremy
Status: student
Grade: 9-12
Location: ME
Date: July 2008

I've been perusing the myriad of acids known to man, and I have become flabbergasted at the dizzying number of varieties. Furthermore, I cannot understand how acids are classified. Could one of your knowledgeable scientists please explain to me what categories acids fall into? If possible, I would like to know every category and subcategory of the acid family tree. Thank you in earnest for your efforts. This is a marvelous site, and I hope it will continue to flourish as it nourishes eager minds.


I do not tend to think of acids as belonging to some kind of classification "tree". Rather, I tend to differentiate acids depending on what they do or what molecular structure they might contain.

The most well-known and often used differentiation is that of strong versus weak acids. Strong acids are those that fully dissociate in water: HA --> H(+) + A(-); where A(-) is the counterion of the the proton - versus those acids that form an equilibrium with their ions (that is, some HA still exists in solution when HA <=> H(+) + A(-).

Another distinction are acids that contain oxygen in their molecular formula (HClO4, HCOOH, etc.) as opposed to those that do not (HCl, HCN, etc.) This is often done since acid strengths can often be trended depending on how the oxygen is structured within the molecule.

There is also the distinction between organic acids versus inorganic acids. Inorganic acids are derived from minerals whereas organic acids are derived from living organisms.

I hope that helped.

Greg (Roberto Gregorius)

Dear Jeremy,

You clearly are interested in Chemistry. I congratulate you on your good taste. If you do not already have a copy of Zumdahl's "College Chemistry" test book, get one. Go to Barnes and Noble, or Amazon and look up choices on their "Used and Out-of-Print" books. Pick the latest edition and cheapest price. Zumdahl can help you answer many of your chemistry questions.

Now about acids. There are 5 common acids: hydrochloric, nitric, sulfuric, phosphoric and acetic. Sulfuric acid is the most important chemical ingredient in the world. Its use, volume and price is sometimes a good indicator of economic health of the USA and world.

Sulfuric, nitric and hydrochloric acids are considered strong mineral acids. Their sources are sulfate, nitrate and chloride salts/minerals. They are strong because each one will easily release a proton in an aqueous solution.

Phosphoric acid is a weak mineral acid. While it has 3 protons, it does not give up the first proton 100% of the time when in water. Here is where the ideas of pH come in.

You have acetic acid at home. It is called vinegar. Acetic acid is a simple organic acid. Its structure is built on the two carbon atoms bonded to each other. One of the carbons has 3 hydrogen atoms bonded to it. These hydrogens are not the protons that make acetic acid an acid. The second carbon has 2 oxygen atoms bonded to it. One of the carbon-oxygen bonds is a double bond; the other carbon-oxygen bond is a single bond. This singly-bonded oxygen atom is also bonded to a hydrogen atom. This hydrogen is the ACIDIC PROTON. It is the proton that dissociates from the molecule and makes vinegar sour. The organic acid group is sometimes written as COOH. All organic acids have this functional group. When an organic chemist wants to add an acid group to a molecule, he runs a carboxylation reaction. When he wants to remove a acid group from an organic acid, he runs a decarboxylation reaction.

The collection of organic acids is vast. Thousands of new ones are synthesized every year. One important subset of organic acids is a group called amino acids. You should have heard of them in biology. They have at least one -COOH group in their structure; they also have a second functional group called the amino group. Its represented by -NH2. The nitrogen is bonded to the 2 hydrogens and a third bond is usually to a carbon atom. Google "Amino acid." You should see diagrams of the amino acids, especially the 20 (or so essential) amino acids. Recall that the amino acids bond to one another to produce proteins.

Read some more, then call us with more questions.

Warren Young

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