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Jump to one of the following inorganic nomenclature parts:   Choose One Introduction to Inorganic Nomenclature Part 1- Molecular Compounds Part 2- Monatomic Ions Part 3- The Stock System Part 4- Polyatomic Ions Part 5- Ionic Compounds Part 6- Binary Acids and Oxoacids Part 7- Quizzes

An acid is something that reacts in water to yield the H₃O¹⁺ ion. I have a very low-brow way of remembering what an acid is. It's a "H-something" compound. This may sound a bit silly but it's basically true (few things are true 100% of the time). For acids, the "H" (hydrogen) is written first to specifically indicate that the compound is an acid. HC₂H₃O₂ is acetic acid and looks like "H-something". You should be able to see that there is more than one hydrogen in acetic acid (there are 4 of them) and the one in front is special. That's about all I want to say as to why it's written that way for right now.

On a side note, there are alternate ways to write acids. Acetic acid is also a carboxylic acid and can be written CH₂COOH. I won't be writing them that way however.

There is a way to test a compound to see if it is acidic (acts as an acid). Litmus is a chemical indicator that can change color. If it is exposed to an acidic solution, it will turn red. Different color papers can be soaked in litmus. If you use blue paper and then expose this treated paper to an acid, the paper turns from blue to red. In the picture below, the left side shows blue litmus paper before and then the right side shows the paper after they have been exposed to an acid.

Let's start with the binary acids. Binary acids are those that have hydrogen and one other element in them. Recall that binary means 2 types of elements in the compound and not necessarily only 2 total atoms. Something a little different about naming them is that they have one name when they are not in water and the acid name when placed in water. For example, HCl in the gas phase is simply hydrogen chloride as we previously learned. HCl in water, however, has the acid name hydrochloric acid.

So, to name the binary acids, you start with the name of the nonhydrogen element chlorine, and then modify it. You add the prefix hydro to the root, chlor in this case, add ic to the end, and then tag on the word "acid" to finish it off. Hydrochloric acid. As for going in the reverse, name to formula, the first tip is if you have the prefix hydro which tells you it's a binary acid. Hydroselenic acid is a binary acid so it's H-something. The "selenic" comes from selenium, and since selenium is in group VIA you need to remember that the formula is H₂Se. Here's more for you to see-

Let me make a quick comment on hydrosulfuric acid. This compound is extremely insoluble in water and readily comes out as a gas. If you live by any body of water like a lake, stream, etc., you may have noticed from time-to-time a rotten egg smell. This can happen during periods when runoff is impeded by debris. Bacteria build up and they produce hydrogen sulfide as a by-product which causes the awful stink. It doesn't really exist in large amounts as an acid but it's mentioned in so many texts that I felt the need to include it.

Note that we have the same exceptions for the ending modification that we had when doing molecular compounds with the added exception of sulfur. You keep the entire element name and just add "ic". You do not drop the "ur" as we did for molecular compounds.

And now to finish up with oxoacids. Oxoacids have hydrogen, oxygen, and another element in the chemical formula. Perhaps the most difficult aspect of the oxoacids is not recoginizing them but remembering how to name them. If you see an acid name that has the hydro prefix, it is not an oxoacid. That's the way to tell the difference between the two.

For instance, H₂SO₄. It leads off with hydrogen and it's got oxygen in it along with another element. The root in this case is the sulfate ion, SO₄^-2. Like the binary acids convention, the ending is modified with "ic" (in this case) and "acid" is tagged on at the end. This acid would be named sulfuric acid. Not hydrosulfuric acid which we did previously! There's another modification in addition to "ic" and that's "ous". When to use them?

That's why the root sulfate became sulfuric acid.

The exceptions here are sulfate ion, sulfite ion, and phosphate ion. Acids made from sulfate and sulfite are built from the element name sulfur just like the name for H₂S. The acid made from phosphate ion is built from the element name phophorus and modified to phosphoric acid.

Quizzes 13 and 14 are yours for the trying.