10.5 OXIDES AND HYDROXIDES

Bases and hydroxides of metals consist of oxides and hydroxides of metals. Metallic oxides are compounds formed by the reaction of metals with oxygen. For example, sodium reacts with oxygen to produce sodium oxide (Na₂O). Similarly, magnesium on ignition in air burns producing magnesium oxide (MgO).

Metal + Oxygen → Metal oxide
4Na + O₂ → 2 Na₂O
2 Mg + O₂ → 2 MgO

When metal oxides dissolve in water, they result in metal cations and oxide ions in aqueous solution. Because oxide ions are unstable in water, they immediately accept protons from water molecules and become hydroxide ions, and the water molecules also become hydroxide ions. Which species is the proton donor in this reaction?

OH⁻ + OH⁻ → 2OH⁻
Metal oxide + water → Metal hydroxide
MgO + H₂O → Mg(OH)₂
Na₂O + H₂O → 2 NaOH
CaO + H₂O → Ca(OH)₂

Most metal oxides and hydroxides are very basic in nature. They show the characteristic properties of bases. However, some metal oxides and hydroxides do not dissolve in water but behave in chemical reactions like both acids and bases. Such metal oxides and hydroxides are called amphoteric oxides and hydroxides, respectively. For example, aluminium oxide (Al₂O₃), aluminium hydroxide (Al(OH)₃), zinc oxide (ZnO), and zinc hydroxide (Zn(OH)₂).

KEY POINTS

  • According to Arrhenius theory, an acid is a substance that ionizes in water to produce H⁺ ions, and a base is a substance that ionizes in water to produce OH⁻ ions.
  • A Bronsted-Lowry acid is a proton donor and a base is a proton acceptor.
  • A strong acid completely dissociates in aqueous solution, and a weak acid dissociates partially in aqueous solution.
  • A strong base completely dissociates in aqueous solution, and a weak base dissociates partially in aqueous solution.
  • Bases are oxides and hydroxides of metals.
  • Alkalis are water-soluble bases.
  • Acids neutralize bases to form salt and water.
  • Acids decompose carbonates to form salt, water, and carbon dioxide.
  • Acids corrode metals and form salt and hydrogen.

References for Additional Information

  • Longman Chemistry for IGCSE.
  • IGCSE Chemistry.
  • Cambridge IGCSE Chemistry.
  • Theories of Acids and Base Chemiguide.