Acids, bases and salts

What makes copper (II) sulphate a salt?

1. Acids, bases and salts: from zero

2. Not all salts are for food

Table salt, NaCl, is a salt. So is copper (II) sulphate,

CuSO₄ C u S O_{4}

, and nobody eats it.

Pick the right answer

Predict: what do NaCl and

CuSO₄ C u S O_{4}

have in common?

A. Each has a metal joined to the rest of an acid
B. Both taste salty
C. Both dissolve in water

Explanation

In both, a metal has taken the place of the hydrogen of an acid: Na with the Cl of hydrochloric acid, Cu with the

SO₄ S O_{4}

of sulphuric acid. That is what makes them salts, not their taste or dissolving.

3. Acids

Acid

A substance that gives positive hydrogen ions (

H⁺ H^{+}

) when dissolved in water.

An ion is an atom, or a group of atoms, with an electric charge. You'll study ions later in Chapter 1.

Acid	Formula	Its salts are called
hydrochloric acid	$HCl H C l$	chlorides
sulphuric acid	$H₂SO₄ H_{2} S O_{4}$	sulphates
nitric acid	$HNO₃ H N O_{3}$	nitrates

Common acids

Acids taste sour and turn litmus red. Never taste them in the lab!

4. Bases

Base

A substance that reacts with an acid to give a salt and water.

Many metal oxides and metal hydroxides are bases. Examples: copper (II) oxide, CuO, and magnesium hydroxide,

Mg(OH)₂ M g (O H)_{2}

Base	Formula	Type
copper (II) oxide	CuO	metal oxide
magnesium hydroxide	$Mg(OH)₂ M g (O H)_{2}$	metal hydroxide

Two bases

You'll meet metal oxides as bases again in the next lesson.

5. Alkalis

Alkali

A base that dissolves in water, giving hydroxide ions (

OH⁻ O H^{-}

Examples: sodium hydroxide, NaOH, and ammonium hydroxide,

NH₄OH N H_{4} O H

. Alkalis turn litmus blue.

True or false?

NaOH is an acid, because its formula contains H.

Explanation

NaOH is an alkali, not an acid: its H is part of the OH group, and in water it gives hydroxide ions (

OH⁻ O H^{-}

6. Acid + base → salt + water

gatheredNaOH + HCl →\\ NaCl + H2Ogathered

An alkali and an acid give a salt and water

Salt

A compound formed when the hydrogen of an acid is replaced by a metal (or ammonium).

Naming rule: the metal's name from the base + the ending from the acid. Copper (II) oxide + sulphuric acid → copper (II) sulphate + water.

7. Name the parent acid

Sort into groups

Which acid does each salt come from?

$ZnCl₂ Z n C l_{2}$ , zinc chloride $MnSO₄ M n S O_{4}$ , manganese (II) sulphate $KNO₃ K N O_{3}$ , potassium nitrate $Na₂SO₄ N a_{2} S O_{4}$ , sodium sulphate $NH₄Cl N H_{4} C l$ , ammonium chloride

Pick the right answer

Which pair reacts to give zinc nitrate and water?

A. ZnO and $HNO₃ H N O_{3}$
B. ZnO and $H₂SO₄ H_{2} S O_{4}$
C. NaOH and $HNO₃ H N O_{3}$
D. ZnO and HCl

Explanation

Zinc nitrate takes its metal, zinc, from the base ZnO, and its ending, nitrate, from nitric acid,

HNO₃ H N O_{3}

. A base and an acid give a salt and water.

8. Summary

Acids give hydrogen ions, $H⁺ H^{+}$ , in water: HCl, $H₂SO₄ H_{2} S O_{4}$ , $HNO₃ H N O_{3}$ .
Bases react with acids to give a salt and water. Many metal oxides and hydroxides are bases.
Alkalis are bases that dissolve in water, giving $OH⁻ O H^{-}$ : NaOH, $NH₄OH N H_{4} O H$ .
Chlorides, sulphates and nitrates come from HCl, $H₂SO₄ H_{2} S O_{4}$ and $HNO₃ H N O_{3}$ .

Back to the hook: NaCl and

CuSO₄ C u S O_{4}

are both salts. In each, a metal replaced the hydrogen of an acid.

Can you explain why

CuSO₄ C u S O_{4}

is a salt of sulphuric acid?

Reveal

CuSO₄ C u S O_{4}

, copper has replaced the hydrogen of sulphuric acid,

H₂SO₄ H_{2} S O_{4}

. The

SO₄ S O_{4}

group comes from the acid, so the salt is a sulphate.

Concepts in this lesson