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WASSCE Chemistry · Acids, Bases, and pH

Acids and bases: definitions, properties and strength

By the WASSCE Tutor team

Updated 12 November 2026 · 9 min read · Prepared against the WASSCE syllabus.

Lime juice, vinegar and the sting of an ant are acidic; wood ash, soap and baking soda are alkaline. Acids and bases are among the most common chemicals in the home, the farm and industry. This lesson defines them in three ways, describes their properties and explains the difference between a strong acid and a concentrated one, a distinction many candidates miss.

Learning objectives

By the end of this lesson you should be able to:

  • define acids and bases using the Arrhenius, Brønsted–Lowry and Lewis theories
  • identify conjugate acid–base pairs
  • describe the properties and reactions of acids and of alkalis
  • define basicity and give examples of mono-, di- and tribasic acids
  • distinguish strong from weak, and concentrated from dilute, acids and bases
  • describe amphoteric oxides and hydroxides

Three definitions

TheoryAcidBaseExample
ArrheniusProduces H⁺ (H₃O⁺) ions in water as the only positive ionsProduces OH⁻ ions in waterHCl → H⁺ + Cl⁻; NaOH → Na⁺ + OH⁻
Brønsted–LowryA proton (H⁺) donorA proton acceptorNH₃ + H₂O ⇌ NH₄⁺ + OH⁻
LewisAn electron-pair acceptorAn electron-pair donorNH₃ (base) donates its lone pair to BF₃ (acid)

The Arrhenius theory works only in water. Brønsted–Lowry explains why ammonia is a base: it accepts a proton.

Conjugate acid–base pairs

When an acid donates a proton, what remains is its conjugate base; when a base accepts a proton, it becomes its conjugate acid.

HCl+H2O→H3O++Cl−\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-
HCl+H2O→H3O++Cl−\begin{gathered}\text{HCl} + \text{H}_2\text{O} \\[6pt] \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-\end{gathered}
  • HCl (acid) and Cl⁻ (its conjugate base)
  • H₂O (base) and H₃O⁺ (its conjugate acid)

Water can act as both an acid and a base: it is amphiprotic.

Properties of acids

  1. Sour taste (never taste laboratory chemicals).
  2. Turn blue litmus red.
  3. React with reactive metals to give a salt and hydrogen: Zn + H₂SO₄ → ZnSO₄ + H₂
  4. React with carbonates and hydrogencarbonates to give a salt, water and carbon(IV) oxide (effervescence): CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
  5. React with bases to give a salt and water only (neutralisation): CuO + H₂SO₄ → CuSO₄ + H₂O
  6. Their aqueous solutions conduct electricity, because they contain ions.

Acids need water. Dry hydrogen chloride gas, or HCl dissolved in methylbenzene, does not turn dry blue litmus red and does not react with magnesium, because it does not ionise to form H⁺ ions without water.

Basicity

Definition

The basicity of an acid is the number of replaceable hydrogen ions that one molecule of the acid can produce.

BasicityAcidIonisation
MonobasicHydrochloric acid HCl, nitric acid HNO₃, ethanoic acid CH₃COOHHCl → H⁺ + Cl⁻
DibasicSulphuric acid H₂SO₄, carbonic acid H₂CO₃H₂SO₄ → 2H⁺ + SO₄²⁻
TribasicPhosphoric(V) acid H₃PO₄H₃PO₄ → 3H⁺ + PO₄³⁻

Ethanoic acid, CH₃COOH, has four hydrogen atoms but is monobasic: only the hydrogen of the –COOH group is replaceable.

Bases and alkalis

Definition

A base is a substance that reacts with an acid to form a salt and water only (metal oxides and hydroxides). An alkali is a base that dissolves in water to give OH⁻ ions.

All alkalis are bases, but not all bases are alkalis. Copper(II) oxide is a base but does not dissolve in water; sodium hydroxide is a base and an alkali.

Common alkalis: sodium hydroxide, potassium hydroxide, calcium hydroxide (lime water) and aqueous ammonia.

Properties of alkalis:

  1. Bitter taste and a soapy feel.
  2. Turn red litmus blue.
  3. Neutralise acids: NaOH + HCl → NaCl + H₂O
  4. React with ammonium salts on warming to give ammonia: NH₄Cl + NaOH → NaCl + H₂O + NH₃
  5. Precipitate insoluble metal hydroxides from solutions of metal salts: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) (blue)

Strength and concentration

StrongWeak
Completely ionised in waterPartly ionised: an equilibrium
Acids: HCl, H₂SO₄, HNO₃Acids: ethanoic acid (vinegar), carbonic acid, citric acid
Bases: NaOH, KOHBases: aqueous ammonia

Hydrochloric acid ionises completely:

HCl(aq)→H+(aq)+Cl−(aq)\text{HCl(aq)} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}
HCl(aq)→H+(aq)+Cl−(aq)\begin{gathered}\text{HCl(aq)} \\[6pt] \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}\end{gathered}

Ethanoic acid ionises only partly; the ⇌ sign shows an equilibrium:

CH3COOH⇌CH3COO−+H+\text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+
CH3COOH⇌CH3COO−+H+\begin{gathered}\text{CH}_3\text{COOH} \\[6pt] \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+\end{gathered}

Concentration is a different idea. It is how much acid is dissolved per dm³.

  • A concentrated solution has a lot of solute per dm³; a dilute one has little.
  • You can have a dilute strong acid (0.01 mol/dm³ HCl) or a concentrated weak acid (glacial ethanoic acid).
Worked example

Comparing equal concentrations of HCl and CH₃COOH (both 0.1 mol/dm³).

Test0.1 mol/dm³ HCl0.1 mol/dm³ CH₃COOH
pHAbout 1About 3
Electrical conductivityHighLow
Rate of reaction with magnesiumFast, vigorous fizzingSlow
Volume of H₂ with excess MgSameSame (all the acid eventually reacts)

The strong acid has more H⁺ ions at any moment, so it reacts faster. Both contain the same total amount of acid, so they give the same total volume of hydrogen.

Amphoteric oxides and hydroxides

Definition

An amphoteric oxide or hydroxide reacts with both acids and alkalis to form salts.

Examples: zinc oxide, aluminium oxide, lead(II) oxide and their hydroxides.

ZnO+2HCl→ZnCl2+H2O\text{ZnO} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\text{O}
ZnO+2HCl→ZnCl2+H2O\begin{gathered}\text{ZnO} + 2\text{HCl} \\[6pt] \rightarrow \text{ZnCl}_2 + \text{H}_2\text{O}\end{gathered}
ZnO+2NaOH+H2O→Na2Zn(OH)4\text{ZnO} + 2\text{NaOH} + \text{H}_2\text{O} \rightarrow \text{Na}_2\text{Zn(OH)}_4
ZnO+2NaOH+H2O→Na2Zn(OH)4\begin{gathered}\text{ZnO} + 2\text{NaOH} + \text{H}_2\text{O} \\[6pt] \rightarrow \text{Na}_2\text{Zn(OH)}_4\end{gathered}

The second product is sodium zincate.

Classes of oxides:

  • acidic oxides (non-metal oxides): CO₂, SO₂, P₄O₁₀
  • basic oxides (metal oxides): Na₂O, CaO, CuO
  • amphoteric oxides: ZnO, Al₂O₃, PbO
  • neutral oxides: CO, NO, N₂O, H₂O
In Sierra Leone

Acids and bases are all around Sierra Leonean homes:

  • Lime and lemon juice contain citric acid, and sour palm wine contains ethanoic acid.
  • Ant and bee stings contain methanoic acid.
  • Wood ash, used in traditional soap making, is alkaline because it contains potassium carbonate.
  • Lime (calcium hydroxide) is used to whitewash walls and to reduce the acidity of farm soil.
Common mistake

"Strong" does not mean "concentrated". Strong describes how completely an acid ionises; concentrated describes how much acid is dissolved per dm³.

Key points

  • Arrhenius: acids give H⁺, bases give OH⁻ in water. Brønsted–Lowry: acids donate protons, bases accept them. Lewis: acids accept electron pairs.
  • Acids turn blue litmus red and react with metals (H₂), carbonates (CO₂) and bases (salt + water); they need water to show acidity.
  • Basicity = number of replaceable H⁺ per molecule: HCl 1, H₂SO₄ 2, H₃PO₄ 3.
  • An alkali is a soluble base.
  • Strong = fully ionised; weak = partly ionised. Concentration is a separate idea.
  • Amphoteric oxides (ZnO, Al₂O₃, PbO) react with both acids and alkalis.

WASSCE examination focus

WAEC tests this

Expect to define acids and bases (especially Brønsted–Lowry, with conjugate pairs), to give reactions of acids with equations, to define basicity, to distinguish strong/weak from concentrated/dilute, and to write equations showing amphoteric behaviour of ZnO or Al₂O₃.

Quick check

  1. Identify the conjugate base of H₂SO₄ in H₂SO₄ + H₂O → H₃O⁺ + HSO₄⁻.
  2. What is the basicity of ethanoic acid? Explain.
  3. Distinguish a dilute strong acid from a concentrated weak acid, with an example of each.
  4. Write an equation to show that aluminium oxide is amphoteric (reaction with acid).

Answers:

  1. HSO₄⁻.
  2. One: only the hydrogen of the –COOH group ionises.
  3. A dilute strong acid is fully ionised but has little acid per dm³ (0.01 mol/dm³ HCl); a concentrated weak acid has a lot of acid per dm³ but is only partly ionised (glacial ethanoic acid).
  4. Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O (it also reacts with NaOH to form sodium aluminate).

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