6.5 Naming Acids

Learning Objectives

By the end of this section, you will be able to:

  • Define acid.
  • Name a binary acid and an oxyacid.

There is one other group of compounds that is important to us—acids—and these compounds have interesting chemical properties. Initially, we will define an acid as an ionic compound of the H+ cation dissolved in water. To indicate that something is dissolved in water, we will use the phase label (aq) next to a chemical formula (where aq stands for “aqueous,” a word that describes something dissolved in water). If the formula does not have this label, then the compound is treated as a molecular compound rather than an acid.

Binary Acids

Some compounds containing hydrogen are members of an important class of substances known as acids. The chemistry of these compounds is explored in more detail in later chapters of this text, but for now, it will suffice to note that many acids release hydrogen ions, H+, when dissolved in water. To denote this distinct chemical property, a mixture of water with acid is given a name derived from the compound’s name. To indicate that something is dissolved in water, we will use the phrase label (aq) next to a chemical formula (where aq stands for “aqueous,” a word that describes something dissolved in water). If the formula does not have this label, then the compound is treated as a molecular compound rather than an acid. If the compound is a binary acid (comprised of hydrogen and one other nonmetallic element):

  1. The word “hydrogen” is changed to the prefix hydro-
  2. The other nonmetallic element name is modified by adding the suffix –ic
  3. The word “acid” is added as a second word

A flowchart summarizes naming acids (Figure 6.5a).

 

The top heading in flowchart reads “Acid”. If the anion of the acid does not contain oxygen, use the prefix "hydro" and the suffix "ic acid". If the anion of the acid contains oxygen, then determine if the anion name ends in "-ite" or "-ate". If the anion name ends in "-ite", name the anion and change the ending to "-ous acid". If the anion name ends in "-ate", name the anion and change the ending to "-ic acid".
Figure 6.5a Flowchart for naming acids. (credit: Chemistry (Open Stax) CC BY 4.0. / Adapted into a flow chart by Revathi Mahadevan. )

For example, when the gas HCl (hydrogen chloride) is dissolved in water, the solution is called hydrochloric acid. Several other examples of this nomenclature are shown in Table 6.5a.

Table 6.5a Names of Some Simple Acids
Name of Gas Name of Acid
HF(g), hydrogen fluoride HF(aq), hydrofluoric acid
HCl(g), hydrogen chloride HCl(aq), hydrochloric acid
HBr(g), hydrogen bromide HBr(aq), hydrobromic acid
HI(g), hydrogen iodide HI(aq), hydroiodic acid
H2S(g), hydrogen sulfide H2S(aq), hydrosulfuric acid

Oxyacids

Many compounds containing three or more elements (such as organic compounds or coordination compounds) are subject to specialized nomenclature rules that you will learn later. However, we will briefly discuss the important compounds known as oxyacids, compounds that contain hydrogen, oxygen, and at least one other element, and are bonded in such a way as to impart acidic properties to the compound (you will learn the details of this in a later chapter). Typical oxyacids consist of hydrogen combined with a polyatomic, oxygen-containing ion. To name oxyacids:

  1. Omit “hydrogen”
  2. Start with the root name of the anion
  3. Replace –ate with –ic, or –ite with –ous
  4. Add “acid”

For example, consider H2CO3 (which you might be tempted to call “hydrogen carbonate”). To name this correctly, “hydrogen” is omitted; the –ate of carbonate is replaced with –ic; and acid is added—so its name is carbonic acid. Other examples are given in Table 6.5b. There are some exceptions to the general naming method (e.g., H2SO4 is called sulfuric acid, not sulfic acid, and H2SO3 is sulfurous, not sulfous, acid).

Table 6.5b Names of Common Oxyacids
Formula Anion Name Acid Name
HC2H3O2 acetate acetic acid
HNO3 nitrate nitric acid
HNO2 nitrite nitrous acid
HClO4 perchlorate perchloric acid
H2CO3 carbonate carbonic acid
H2SO4 sulfate sulfuric acid
H2SO3 sulfite sulfurous acid
H3PO4 phosphate phosphoric acid

Example 6.5a

Problems

Name each acid without consulting Table 6.6b.

  1. HBr
  2. H2SO4

Solutions

  1. As a binary acid, the acid’s name is hydro- + stem name + -ic acid. Because this acid contains a bromine atom, the name is hydrobromic acid.
  2. Because this acid is derived from the sulfate ion, the name of the acid is the stem of the anion name + -ic acid. The name of this acid is sulfuric acid.

Exercise 6.5a

Name each acid.

  1. HF
  2. HNO2

Check Your Answer[1]

All acids have some similar properties. For example, acids have a sour taste; in fact, the sour taste of some of our foods, such as citrus fruits and vinegar, is caused by the presence of acids in food. Many acids react with some metallic elements to form metal ions and elemental hydrogen. Acids make certain plant pigments change colours; indeed, the ripening of some fruits and vegetables is caused by the formation or destruction of excess acid in the plant. In a later chapter, we will explore the chemical behaviour of acids.

Acids are very prevalent in the world around us. We have already mentioned that citrus fruits contain acid; among other compounds, they contain citric acid, H3C6H5O7(aq). Oxalic acid, H2C2O4(aq), is found in spinach and other green leafy vegetables. Hydrochloric acid not only is found in the stomach (stomach acid) but also can be bought in hardware stores as a cleaner for concrete and masonry. Phosphoric acid is an ingredient in some soft drinks.

Links to Interactive Learning Tools

Attribution & References

Except where otherwise noted, this page is adapted by Adrienne Richards from:

Adaptations include combining section 2.7, 4.3 and Chapter 3.


    1. hydrofluoric acid
    2. nitrous acid
definition

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6.5 Naming Acids Copyright © 2023 by Gregory Anderson; Caryn Fahey; Jackie MacDonald; Adrienne Richards; Samantha Sullivan Sauer; J.R. van Haarlem; and David Wegman is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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