Chemistry · Lesson 7

Acids, bases and pH

JAMBWAECNECO

What the syllabus asks

JAMB: Acids, bases and salts

WAEC: Acids, bases and salts: definitions (Arrhenius, Brønsted–Lowry), properties, strong and weak acids, pH scale, indicators, neutralisation, preparation and types of salts, hydrolysis, acid–base titration

NECO: Acids, bases and salts: definitions (Arrhenius, Brønsted–Lowry), properties, strong and weak acids, pH scale, indicators, neutralisation, preparation and types of salts, hydrolysis, acid–base titration

What you will learn

Define acids, bases and alkalis (Arrhenius and Brønsted–Lowry).
Distinguish strong from weak and concentrated from dilute.
Use the pH scale and indicators.
Write equations for the typical reactions of acids.
Classify salts and choose a method to prepare a given salt.
Explain salt hydrolysis and perform titration calculations.

Key points

Arrhenius acid gives H⁺ in water; Brønsted–Lowry acid = proton donor, base = proton acceptor.
An alkali is a soluble base giving OH⁻.
Strong/weak = degree of ionisation; concentrated/dilute = amount per dm³.
pH = −log[H⁺]; pH + pOH = 14; each pH unit = 10× change.
Acid + metal → salt + H₂; + base → salt + water; + carbonate → salt + water + CO₂.
Salt preparation: precipitation (insoluble), titration (Na/K/NH₄), excess base (others).
Hydrolysis: the stronger parent decides; titration uses C × V and the mole ratio.
Oxides: non-metal oxides are mostly acidic, metal oxides basic; ZnO, Al₂O₃ and PbO are amphoteric; CO, NO and N₂O are neutral.
Hydrated salts lose water on heating; efflorescent salts lose water to air, hygroscopic substances absorb it, deliquescent ones dissolve in it.

Start here: What makes an acid an acid?

The word "acid" comes from the Latin acidus, meaning sour. Taste unripe mango, lime or vinegar and you know acids. But tasting is dangerous in a laboratory, so chemists use better tests and definitions.

Arrhenius definition (1884). An acid is a substance that produces hydrogen ions, H⁺, as the only positive ions when dissolved in water. For example: HCl(aq) → H⁺(aq) + Cl⁻(aq).

A bare H⁺ ion is just a proton, too reactive to exist alone. In water it attaches to a water molecule, forming the hydroxonium ion, H₃O⁺: HCl + H₂O → H₃O⁺ + Cl⁻. Exams accept H⁺(aq) as shorthand.

Why water matters. Dry hydrogen chloride gas, or HCl dissolved in methylbenzene (an organic solvent), does NOT turn dry blue litmus red and does not conduct electricity. Without water there is no ionisation, so it shows no acidic properties. This is a classic WAEC question.

Properties of acids: sour taste; turn blue litmus red; pH below 7; react with reactive metals to give hydrogen; react with carbonates to give CO₂; neutralise bases.

Worked example 7.1

Write ionisation equations for (a) nitric acid and (b) sulphuric acid, and state the basicity of each.

(a) HNO₃(aq) → H⁺(aq) + NO₃⁻(aq). One replaceable H, so it is monobasic.
(b) H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq). Two replaceable H, so it is dibasic.

monobasic; dibasic

⚠️ Common mistake: counting all the H atoms for basicity. Ethanoic acid, CH₃COOH, has four H atoms but only ONE replaceable (the one in COOH), so it is monobasic.

Try it yourself. What is the basicity of phosphoric(V) acid, H₃PO₄?

Show solution

Tribasic (3 replaceable H⁺).

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Practice questions

1. A solution with pH 2 is

strongly acidic
weakly acidic
neutral
strongly alkaline

Show answer

A: strongly acidic
The lower the pH below 7, the stronger the acid.

2. Which gas is produced when dilute HCl reacts with zinc?

Oxygen
Chlorine
Carbon(IV) oxide
Hydrogen

Show answer

D: Hydrogen
Metal + acid → salt + hydrogen.

3. The salt formed when sulphuric acid reacts with sodium hydroxide is

sodium sulphite
sodium hydrogen carbonate
sodium sulphate
sodium sulphide

Show answer

C: sodium sulphate
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.

4. A suitable indicator for titrating ethanoic acid with sodium hydroxide is

universal indicator
phenolphthalein
methyl orange
litmus

Show answer

B: phenolphthalein
Weak acid + strong base → end point in alkaline range → phenolphthalein.

5. The pH of a 0.01 mol/dm³ HCl solution is

2
1
0.01
12

Show answer

A: 2
pH = −log[H⁺] = −log(10⁻²) = 2.

Key words

Acid: A substance that produces hydrogen ions (H⁺) as the only positive ions in water.
Base: A substance that reacts with an acid to form a salt and water only.
Alkali: A base that dissolves in water, giving hydroxide ions.
Proton donor: A Brønsted–Lowry acid.
Strong acid: An acid that is completely ionised in water.
Weak acid: An acid that is only partially ionised in water.
Concentration: The amount of solute per unit volume (mol dm⁻³).
pH: A measure of acidity: −log₁₀[H⁺].
Indicator: A substance whose colour changes with pH.
Neutralisation: The reaction of an acid with a base to form a salt and water.
Salt: A compound formed when the H⁺ of an acid is replaced by a metal or ammonium ion.
Hydrolysis: The reaction of a salt with water, giving an acidic or alkaline solution.

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