C. Chemistry: Chemical Reactions · Chemical Reactions

Naming Acids and Classifying Compounds

C3.8 C2.1 A1.12About 65 minutes

Learning goals

I can explain what (aq) means and why an acid must be dissolved in water.
I can name acids from their formulas using the -ide, -ate and -ite rules.
I can write formulas for common acids.
I can classify a compound as ionic, molecular or an acid from its formula.

Words to know

acid
A compound that produces hydrogen ions (H⁺) when it dissolves in water.

aqueous (aq)
Dissolved in water.

state symbol
(s), (l), (g) or (aq) written after a formula to show its state.

binary acid
An acid made of hydrogen and one nonmetal, e.g. HCl(aq); named hydro-...-ic acid.

oxyacid
An acid containing hydrogen, oxygen and another element, e.g. H₂SO₄(aq).

hydrogen ion
H⁺, a hydrogen atom that has lost its electron; acids release it in water.

Acids are everywhere

Your stomach makes hydrochloric acid to help digest food. A car battery contains sulfuric acid. Some colas contain phosphoric acid, and sparkling water contains a little carbonic acid. Farmers use fertilizers made from nitric acid. Acids are among the most important chemicals in industry and in your body.

In this lesson you will learn what makes a compound an acid (for naming purposes), how to name acids and write their formulas, and how to look at any formula and decide whether it is ionic, molecular or an acid. In the pH lesson later in this unit you will study how acids behave.

State symbols and (aq)

Chemists often add a state symbol in brackets after a formula:

(s) solid, (l) liquid, (g) gas
(aq) aqueous: dissolved in water ("aqua" is Latin for water)

This matters for acids. Pure hydrogen chloride, HCl(g), is a gas made of molecules. It is not called an acid. When it dissolves in water, the molecules break apart into H⁺ ions and Cl⁻ ions. Now the solution, HCl(aq), is hydrochloric acid.

A simple definition you can use for now: an acid is a compound that produces hydrogen ions, H⁺, when it dissolves in water. That is why acid formulas are written with H first. (In fact, each H⁺ attaches to a water molecule to make H₃O⁺, the hydronium ion. You will meet this idea again in Grade 11 chemistry.)

Hydrogen chloride gas is made of HCl molecules. In water it separates into H⁺ and Cl⁻ ions, making hydrochloric acid. The H⁺ ions are what make a solution acidic.

Naming acids: it all depends on the negative ion

Every acid is hydrogen ions (H⁺) plus a negative ion. To name the acid, look at the name of the negative ion and use one of three rules.

Rule 1: -ide ions (no oxygen). Use hydro- + stem + -ic acid.

Cl⁻ chloride → HCl(aq) hydrochloric acid
Br⁻ bromide → HBr(aq) hydrobromic acid
F⁻ fluoride → HF(aq) hydrofluoric acid
S²⁻ sulfide → H₂S(aq) hydrosulfuric acid

Rule 2: -ate ions. Change -ate to -ic acid. No "hydro".

NO₃⁻ nitrate → HNO₃(aq) nitric acid
SO₄²⁻ sulfate → H₂SO₄(aq) sulfuric acid
PO₄³⁻ phosphate → H₃PO₄(aq) phosphoric acid
CO₃²⁻ carbonate → H₂CO₃(aq) carbonic acid
CH₃COO⁻ acetate → CH₃COOH(aq) acetic acid (vinegar; this one is written with its H at the end)

Rule 3: -ite ions. Change -ite to -ous acid.

NO₂⁻ nitrite → HNO₂(aq) nitrous acid
SO₃²⁻ sulfite → H₂SO₃(aq) sulfurous acid
ClO⁻ hypochlorite → HClO(aq) hypochlorous acid (the germ-killer in pool water)

Notice that sulfur and phosphorus get a little extra in the middle: sulfuric, not "sulfic"; phosphoric, not "phosphic". These just sound better and are the accepted names.

The ending of the negative ion decides the acid's name. -ide gets hydro- and -ic; -ate becomes -ic; -ite becomes -ous.

A memory trick

"I ate something icky, but the bite was delicious."

-ate → -ic, and -ite → -ous. If there is no oxygen (an -ide ion), add hydro-.

Writing acid formulas

The acid has to be neutral overall, so the number of H⁺ ions equals the size of the negative ion's charge.

Nitrate is 1−, so nitric acid has one H: HNO₃.
Sulfate is 2−, so sulfuric acid has two H: H₂SO₄.
Phosphate is 3−, so phosphoric acid has three H: H₃PO₄.

No brackets are needed: H₂SO₄, not H₂(SO₄). Add (aq) to show the acid is dissolved in water.

Ionic, molecular or acid? Classifying any compound

Expectation C3.8 asks you to identify compounds and write their formulas using the periodic table and a list of polyatomic ions. Put everything from the last three lessons together:

Acid: formula written with H first, dissolved in water (aq). HCl(aq), HNO₃(aq), H₂SO₄(aq).
Ionic: contains a metal (or the ammonium ion, NH₄⁺) combined with a nonmetal or a polyatomic ion. NaCl, KNO₃, NaOH, NH₄Cl, CuSO₄.
Molecular: nonmetals only, not an acid. CO₂, H₂O, NH₃, CH₄, N₂O.

A few compounds start with H but are not acids. Water, H₂O, is molecular and neutral. Hydrogen peroxide, H₂O₂, is usually treated as molecular.

A quick way to classify a compound from its formula. Ask the questions in order.

Worked example

Example 1: Name an acid

Name (a) HBr(aq), (b) H₂SO₃(aq), (c) HClO₃(aq).

(a) The negative ion is Br⁻, bromide (an -ide ion). Rule 1: hydro + brom + ic → hydrobromic acid.
(b) The negative ion is SO₃²⁻, sulfite (an -ite ion). Rule 3: sulfite → sulfurous acid.
(c) The negative ion is ClO₃⁻, chlorate (an -ate ion). Rule 2: chlorate → chloric acid.

(a) hydrobromic acid; (b) sulfurous acid; (c) chloric acid.

Worked example

Example 2: Write the formula

Write formulas for (a) carbonic acid and (b) hydrosulfuric acid.

(a) "Carbonic" with no hydro- comes from an -ate ion: carbonate, CO₃²⁻. Charge 2−, so two H⁺: H₂CO₃(aq).
(b) "Hydro-...-ic" means an -ide ion: sulfide, S²⁻. Charge 2−, so two H⁺: H₂S(aq).

(a) H₂CO₃(aq); (b) H₂S(aq).

Worked example

Example 3: Classify and name

Classify each compound as ionic, molecular or an acid, and name it: (a) NH₃, (b) H₃PO₄(aq), (c) NH₄NO₃, (d) N₂O₅, (e) Ca(OH)₂.

(a) Nonmetals only, not written with H first: molecular, ammonia.
(b) H first and aqueous: acid. Phosphate is an -ate ion → phosphoric acid.
(c) Contains the ammonium ion: ionic, ammonium nitrate.
(d) Nonmetals only: molecular, dinitrogen pentoxide.
(e) Contains a metal (calcium): ionic, calcium hydroxide.

Molecular: NH₃, N₂O₅. Ionic: NH₄NO₃, Ca(OH)₂. Acid: H₃PO₄(aq).

Hands-on activity

Acid label hunt and sort

You need: food and drink labels from your kitchen (pop, sparkling water, candy, juice, vitamin C tablets, vinegar), and index cards or paper. Safety: read labels only. Do not taste anything for this activity. Do not open cleaning products; reading their labels is enough.

Find every ingredient whose name ends in "acid". Common ones are citric acid (candy, juice), ascorbic acid (vitamin C), acetic acid (vinegar), phosphoric acid (some colas) and carbonic acid (made when carbon dioxide dissolves in sparkling drinks, though it is rarely listed by name).
For acetic, phosphoric and carbonic acid, write the formula using the rules from this lesson.
Citric and ascorbic acid are larger carbon-based molecules. Their names follow rules you will learn in Grade 11 and 12. Just record them.
Make 15 cards: 5 acids, 5 ionic compounds and 5 molecular compounds from this unit (e.g. HNO₃(aq), KCl, CO₂, Na₂CO₃, H₂SO₄(aq), CH₄, NH₄Cl...). Shuffle and have someone time you as you sort them into three piles using the flowchart.

What to look for: acetic acid CH₃COOH(aq), phosphoric acid H₃PO₄(aq), carbonic acid H₂CO₃(aq). In the sort, every acid starts with H and is aqueous; every ionic compound contains a metal or NH₄⁺; every molecular compound has only nonmetals.

Just for fun

Your stomach holds hydrochloric acid with a pH that is often between about 1.5 and 3.5. To avoid digesting itself, it coats its walls with mucus and keeps replacing the cells of its lining every few days. Now that is a serious self-care routine.

Your turn

What does (aq) mean after a formula?

Name HF(aq).

Name HNO₃(aq) and HNO₂(aq).

Write the formula for sulfuric acid.

Write the formula for hydrochloric acid.

Why is HCl(g) not called hydrochloric acid?

Classify as ionic, molecular or acid: (a) KOH, (b) SO₂, (c) H₂CO₃(aq).

More practice: warm-up

What ion do all acids produce in water?

What does -ate change to in an acid name?

What does -ite change to in an acid name?

Which acid is in vinegar?

Name H₂SO₄(aq).

More practice: on your own

Write the formula for phosphoric acid and explain the number of H atoms.

Name HClO₂(aq). (ClO₂⁻ is the chlorite ion.)

Name HI(aq).

A student names H₂S(aq) "sulfuric acid". Explain the mistake.

Classify and name: (a) NH₄Cl, (b) PCl₃, (c) HBr(aq), (d) MgSO₄.

Challenge

Perchloric acid is HClO₄(aq). What is the name and formula of the negative ion it contains? Use the acid naming rules backward.

Sodium hydrogen sulfate, NaHSO₄, contains hydrogen but is not named as an acid. Explain why, and classify it.

Carbonic acid forms when carbon dioxide dissolves in water. Write a word equation and a formula equation for this, and explain why the H count works.

Check your understanding

Quiz

What is the name of HCl(aq)?

What is the formula of nitric acid?

An acid made from an -ite ion has a name ending in...

Which compound is an acid?

How many hydrogen atoms are in one molecule of phosphoric acid?

Which compound is molecular?

For parents and teachers

Big idea: expectation C3.8 lists acids (e.g. HCl(aq), H₂SO₄(aq)) among the compounds students must identify and write formulas for. This lesson gives three naming rules (hydro-...-ic, -ate → -ic, -ite → -ous) and a way to sort any formula into ionic, molecular or acid. The behaviour of acids (pH, indicators) is taught later in this unit, and the hydronium ion is a Grade 11 idea mentioned only briefly.

Common difficulties:

Adding "hydro-" to every acid. Only acids from -ide ions (no oxygen) get hydro-.
Confusing sulfide, sulfite and sulfate (and their acids hydrosulfuric, sulfurous, sulfuric).
Getting the number of H wrong. It equals the size of the negative ion's charge.
Forgetting (aq). Without water, HCl is a gas, not an acid.

How to help: say the memory trick together ("I ate something icky, but the bite was delicious"). Use the card sort as a timed game. The label hunt is read-only: please make sure nothing is tasted or opened, and that cleaning products stay closed.