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2.5.2Inorganic Chemistry

Acids, Alkalis and Neutralisation — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 21 of 52

📚 Edexcel International GCSE Science Double Award 4SD0 - Chemistry Hub / Topical Revision
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Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
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Part 1 of 4Acids, alkalis and the pH scale

Acids, alkalis and the pH scale
Acids
Acids substances that dissolve in water to produce hydrogen ions, H+(aq). (pH less than 7).
Bases
Bases substances that neutralise acids to form a salt and water (metal oxides and metal hydroxides).
Alkalis
Alkalis soluble bases that dissolve in water to produce hydroxide ions, OH-(aq). (pH greater than 7).
Indicators and pH
  • Universal Indicatorred (strong acid), yellow/green (weak acid), green (neutral, pH 7), blue (weak alkali), purple (strong alkali).
  • Litmusred in acid, blue in alkali.
  • Methyl orangered in acid, yellow in alkali.
  • Phenolphthaleincolourless in acid, bright pink in alkali.
Neutralisation ionic equation

H+(aq) + OH-(aq) → H2O(l)

Part 2 of 4Reactions of acids

ReactantsProductsExample Equation
Acid + MetalSalt + HydrogenMg + 2HCl → MgCl2 + H2
Acid + Metal OxideSalt + WaterCuO + H2SO4 → CuSO4 + H2O
Acid + Metal HydroxideSalt + WaterNaOH + HCl → NaCl + H2O
Acid + Metal CarbonateSalt + Water + Carbon DioxideCaCO3 + 2HCl → CaCl2 + H2O + CO2

Part 3 of 4Strong vs weak acids

Strong vs weak acids
Strong acids
Strong acids fully dissociate/ionise in aqueous solution:

HCl(aq) → H+(aq) + Cl-(aq)

Examples: hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3).
Weak acids
Weak acids only partially dissociate/ionise in aqueous solution (reversible reaction at equilibrium):

CH3COOH(aq) ⇌ CH3COO-(aq) + H+(aq)

Examples: ethanoic acid, citric acid, carbonic acid.
Strength vs concentration
  • Strength refers to the degree of ionisation of acid molecules in water.
  • Concentration refers to the amount of acid dissolved in a given volume of solution (mol dm-3 or g dm-3).

Part 4 of 4Core practical: Preparing a soluble salt

  • Preparation of copper sulfate crystals from copper(II) oxide and sulfuric acid:
    • 1. Warm sulfuric acid gently in a beaker over a water bath.
    • 2.
      • Add insoluble copper(II) oxide powder in excess, stirring until no more dissolves (ensures all acid is neutralised).
    • 3. Filter the warm mixture to remove the unreacted excess black copper oxide residue.
    • 4. Heat the clear blue filtrate in an evaporating basin until the crystallisation point is reached.
    • 5. Leave to cool slowly, filter the crystals, and pat dry with filter paper.

Key terms

TermMeaning
AcidA substance that releases H+ ions when dissolved in water.
AlkaliA soluble base that releases OH- ions in water.
NeutralisationThe reaction between H+ and OH- ions to produce neutral water (H2O).
Strong acidAn acid that completely dissociates into ions in aqueous solution.
Weak acidAn acid that only partially dissociates into ions in aqueous solution.

Avoid these mistakes

Avoid these mistakes
Confusing strong and concentrated — a strong acid fully ionises regardless of concentration; a concentrated acid simply contains many moles per unit volume.
Heating copper sulfate crystals to complete dryness — this produces anhydrous white powder instead of hydrated blue crystals; always crystallise by cooling.

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