How to Name Acids: Binary Acids and Oxyacids

Hydrochloric acid, sulfuric acid, nitrous acid, perchloric acid, hypochlorous acid — acid names can look like a random collection of prefixes and endings. In fact they follow a small set of rules that link each acid’s name to the anion it contains. Once you know the pattern, you can name almost any common acid from its formula, or write its formula from its name. This guide sets out the rules step by step, with examples and practice.

What counts as an acid here?

In naming, an acid is usually a compound that contains hydrogen combined with an anion and that releases H⁺ ions in water. Its formula usually starts with H: HCl, H₂SO₄, HNO₃. Acid names are normally used for the substance dissolved in water (aqueous). For example, HCl(g) is “hydrogen chloride”, but HCl(aq) is “hydrochloric acid”.

There are two main families:

1. Binary acids — hydrogen plus one other element (usually a halogen or sulfur): HF, HCl, HBr, HI, H₂S.
2. Oxyacids — hydrogen, oxygen and another element: H₂SO₄, HNO₃, H₃PO₄, HClO₃.

Step 1: identify the anion

Every acid name comes from the name of its anion — the part left when you remove the H⁺ ions.

Acid formula	Anion	Anion name
HCl	Cl⁻	chloride
H₂S	S²⁻	sulfide
HNO₃	NO₃⁻	nitrate
HNO₂	NO₂⁻	nitrite
H₂SO₄	SO₄²⁻	sulfate
H₂SO₃	SO₃²⁻	sulfite
H₃PO₄	PO₄³⁻	phosphate
H₂CO₃	CO₃²⁻	carbonate

The number of hydrogens equals the charge of the anion, so the acid is neutral overall. See common ions and their charges for a full list.

Step 2: apply the rule for the anion’s ending

This is the heart of the system. There are three anion endings, and each gives a different acid name:

Anion ending	Acid name pattern	Memory aid
-ide	hydro___ic acid	“Hydro” for -ide
-ate	___ic acid	“I ate something icky”
-ite	___ous acid	“Spiteful and dangerous”

Binary acids: -ide → hydro-…-ic acid

Formula (aq)	Anion	Acid name
HF	fluoride	hydrofluoric acid
HCl	chloride	hydrochloric acid
HBr	bromide	hydrobromic acid
HI	iodide	hydroiodic acid
H₂S	sulfide	hydrosulfuric acid
HCN	cyanide	hydrocyanic acid

Note that hydrocyanic acid follows the pattern because its anion is cyanide, even though it contains three elements.

Oxyacids: -ate → -ic acid

Formula	Anion	Acid name
HNO₃	nitrate	nitric acid
H₂SO₄	sulfate	sulfuric acid
H₃PO₄	phosphate	phosphoric acid
H₂CO₃	carbonate	carbonic acid
HClO₃	chlorate	chloric acid
CH₃COOH	ethanoate (acetate)	ethanoic acid (acetic acid)

Oxyacids: -ite → -ous acid

Formula	Anion	Acid name
HNO₂	nitrite	nitrous acid
H₂SO₃	sulfite	sulfurous acid
HClO₂	chlorite	chlorous acid

Spelling note: for sulfur and phosphorus, a syllable is added back: sulfuric (not “sulfic”) and phosphoric (not “phosphic”). Nitrogen and carbon don’t need this.

Step 3: handle per- and hypo-

Chlorine, bromine and iodine can form four different oxyanions. Two extra prefixes extend the series:

• per- means one more oxygen than the -ate ion.
• hypo- means one fewer oxygen than the -ite ion.

The prefixes carry straight over to the acid names:

Anion	Formula	Acid formula	Acid name
perchlorate	ClO₄⁻	HClO₄	perchloric acid
chlorate	ClO₃⁻	HClO₃	chloric acid
chlorite	ClO₂⁻	HClO₂	chlorous acid
hypochlorite	ClO⁻	HClO	hypochlorous acid

Hypochlorous acid is the active substance in bleach solutions and is also made by white blood cells to kill bacteria (see the immune system as chemistry). For more detail on these ions, see naming oxyanions.

Going the other way: formula from name

1. Identify the anion from the name:
   • hydro-…-ic acid → -ide anion
   • …-ic acid → -ate anion
   • …-ous acid → -ite anion
2. Find the anion’s formula and charge (learn the common ones).
3. Add enough H⁺ to cancel the charge.

Example: phosphorous acid → -ous → phosphite, PO₃³⁻ → add 3 H → H₃PO₃. (In reality, one of H₃PO₃’s hydrogens is bonded directly to phosphorus and isn’t acidic — a nice reminder that names follow patterns while chemistry can have surprises.)

Example: hydrobromic acid → hydro-…-ic → bromide, Br⁻ → HBr(aq).

Example: chloric acid → -ic → chlorate, ClO₃⁻ → HClO₃.

Organic acids

Carboxylic acids are named from the hydrocarbon chain, replacing -e in the alkane name with -oic acid:

• methane → methanoic acid, HCOOH (traditional name: formic acid, found in ant stings)
• ethane → ethanoic acid, CH₃COOH (acetic acid, in vinegar)
• propane → propanoic acid, CH₃CH₂COOH

Their anions end in -oate: methanoate, ethanoate, propanoate.

Traditional vs IUPAC names

Many acids have long-established traditional names that are still used everywhere — sulfuric, nitric, hydrochloric. IUPAC also defines longer systematic names built from the hydrogen atoms and the anion, but they’re rarely used for common acids — and chemists never say them aloud in the lab. For school exams, use the traditional acid names shown in this guide.

Why the endings make sense

The system may look arbitrary, but it’s consistent. The -ic and -ous endings were originally used to show higher and lower amounts of oxygen in an acid, long before chemists understood ions. Later, when the anions were named, the same idea carried over: the anion from an -ic acid got the ending -ate, and the anion from an -ous acid got -ite. The prefix hydro- was added to acids that contain no oxygen at all, to make clear they’re a different family. So when you see “hydro-”, think “no oxygen”; when you see “-ic” or “-ous”, think “oxyacid”, with -ic having more oxygen than -ous.

This also helps you check an answer. If you’ve written a formula for “hydrosulfuric acid” that contains oxygen, something has gone wrong — the “hydro-” tells you it must be H₂S. If you’ve written chloric acid with fewer oxygens than chlorous acid, you’ve swapped the endings.

Acid strength isn’t in the name

The name tells you the formula, not how strong the acid is. Hydrochloric, nitric and sulfuric acids are strong acids that ionise completely in water, while hydrofluoric, nitrous and carbonic acids are weak and ionise only partly. Within an oxyacid series, though, there’s a pattern: more oxygen atoms usually means a stronger acid, so perchloric acid is far stronger than hypochlorous acid.

Practice

Name these acids:

1. HI(aq)
2. HNO₂
3. H₂SO₄
4. HBrO₄
5. HClO
6. H₃PO₄

Write formulas for: 7. hydrofluoric acid 8. carbonic acid 9. sulfurous acid 10. perchloric acid

Answers:

1. hydroiodic acid (iodide → hydro-…-ic)
2. nitrous acid (nitrite → -ous)
3. sulfuric acid (sulfate → -ic)
4. perbromic acid (perbromate → per-…-ic)
5. hypochlorous acid (hypochlorite → hypo-…-ous)
6. phosphoric acid (phosphate → -ic)
7. HF(aq)
8. H₂CO₃
9. H₂SO₃
10. HClO₄

Common mistakes

• Adding “hydro-” to oxyacids. “Hydrosulfuric acid” is H₂S, not H₂SO₄.
• Mixing up -ic and -ous. -ate → -ic; -ite → -ous.
• Forgetting the charge when writing formulas: sulfate is 2−, so sulfuric acid is H₂SO₄, not HSO₄.
• Naming the gas as an acid. HCl(g) is hydrogen chloride; only HCl(aq) is hydrochloric acid.

Key takeaways

• Start from the anion: the number of H atoms equals the anion’s charge.
• -ide → hydro-…-ic acid (HCl, hydrochloric acid).
• -ate → -ic acid (H₂SO₄, sulfuric acid).
• -ite → -ous acid (HNO₂, nitrous acid).
• per- and hypo- carry over (HClO₄ perchloric; HClO hypochlorous).
• Carboxylic acids end in -oic acid.

For more on acid strength, see the pH scale explained.