Acids, Bases and Salt Preparations

Acids release H⁺ ions

• An acid is a substance that donates a proton (H⁺ ion) when it dissolves in water — a proton donor
• The free hydrogen ion is what makes the solution acidic
• Examples:
  • HCl(aq) → H⁺(aq) + Cl⁻(aq)
  • HNO₃(aq) → H⁺(aq) + NO₃⁻(aq)
  • H₂SO₄(aq) → 2 H⁺(aq) + SO₄²⁻(aq)

Bases accept H⁺ ions

• A base is a substance that can accept a proton — a proton acceptor
• Bases neutralise acids to give a salt plus water
• Typical bases include:
  • Metal oxides (e.g. CuO, MgO)
  • Metal hydroxides (e.g. NaOH, Ca(OH)₂, Fe(OH)₃)
  • Metal carbonates (e.g. CaCO₃, ZnCO₃)
  • Ammonia, NH₃

Bases versus alkalis

• A base that is also soluble in water is called an alkali
• So every alkali is a base, but not every base is an alkali — many bases (CuO, Fe(OH)₃, ZnCO₃) are not water-soluble
• In water, an alkali produces OH⁻ ions; the OH⁻ ions are the proton acceptors
• Common alkalis encountered in the IGCSE syllabus:
  • Potassium hydroxide, KOH — gives K⁺ ions and OH⁻ ions in water
  • Sodium hydroxide, NaOH — gives Na⁺ ions and OH⁻ ions in water
  • Aqueous ammonia, NH₃(aq) — partially reacts with water to give NH₄⁺ ions and OH⁻ ions
• Aqueous ammonia is also called ammonia solution or, historically, "ammonium hydroxide"; the gas itself is NH₃, the ion is NH₄⁺