7.1 The characteristic properties of acids and bases

2026 Syllabus Objectives

Core:

  1. Describe the characteristic properties of acids in terms of their reactions with: (a) metals (b) bases (c) carbonates
  2. Describe acids in terms of their effect on: (a) litmus (b) thymolphthalein (c) methyl orange
  3. State that bases are oxides or hydroxides of metals and that alkalis are soluble bases
  4. Describe the characteristic properties of bases in terms of their reactions with: (a) acids (b) ammonium salts
  5. Describe alkalis in terms of their effect on: (a) litmus (b) thymolphthalein (c) methyl orange
  6. State that aqueous solutions of acids contain H⁺ ions and aqueous solutions of alkalis contain OH⁻ ions
  7. Describe how to compare hydrogen ion concentration, neutrality, relative acidity and relative alkalinity in terms of colour and pH using universal indicator paper
  8. Describe the neutralisation reaction between an acid and an alkali to produce water, H⁺(aq) + OH⁻(aq) → H₂O(l)

Supplement: 9. Define acids as proton donors and bases as proton acceptors 10. Define a strong acid as an acid that is completely dissociated in aqueous solution and a weak acid as an acid that is partially dissociated in aqueous solution 11. State that hydrochloric acid is a strong acid, as shown by the symbol equation, HCl(aq) → H⁺(aq) + Cl⁻(aq) 12. State that ethanoic acid is a weak acid, as shown by the symbol equation, CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)

🔬 What are Acids?

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An acid is a substance that dissolves in water, producing H+(aq)H^+(aq) ions. A solution of an acid turns litmus red and has a pH below 7.

Key Characteristics of Acids 🔑

  • Acids have a sour or sharp taste (though taste should never be used as a test!)
  • Many acids are corrosive - they can dissolve or 'eat away' at other materials such as wood, metals or human skin
  • Acids are found naturally in fruits (citric acid), vinegar (ethanoic acid), and sour milk (lactic acid)

Types of Acids

Acids can be classified into two main categories:

1. Organic Acids

  • Found in animal and plant material
  • Generally weak acids
  • Examples include ethanoic acid, methanoic acid, lactic acid, and citric acid

2. Mineral Acids

  • Often powerful and corrosive
  • Can be strong or weak acids
  • Examples include hydrochloric acid, nitric acid, sulfuric acid, and phosphoric acid

Common Acids and Their Uses

TypeNameFormulaStrong or weak?Where found or used
Organicethanoic acidCH3COOHCH_3COOHweakin vinegar
Organicmethanoic acidHCOOHHCOOHweakin ant and nettle stings; used in kettle descaler
Organiclactic acidCH3CH(OH)CO2HCH_3CH(OH)CO_2Hweakin sour milk
Organiccitric acidC6H8O7C_6H_8O_7weakin lemons, oranges and other citrus fruits
Mineralcarbonic acidH2CO3H_2CO_3weakin fizzy soft drinks
Mineralhydrochloric acidHClHClstrongused in cleaning metal surfaces; found as dilute acid in the stomach
Mineralnitric acidHNO3HNO_3strongused in making fertilisers and explosives
Mineralsulfuric acidH2SO4H_2SO_4strongin car batteries; used in making fertilisers, paints and detergents
Mineralphosphoric acidH3PO4H_3PO_4strongin anti-rust paint; used in making fertilisers

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