Classifying Oxides (Cambridge O Level Chemistry)

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Alexandra Brennan

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Classifying Oxides

  • Oxides are compounds made from one or more atoms of oxygen combined with one other element
  • Examples of oxides include: MgO, ZnO, K2O, CO2, SO2, H2O
  • Oxides can be classified based on their acid-base characteristics

Acidic and basic oxides

  • Acidic and basic oxides have different properties and values of pH
  • The difference in their pH stems from whether they are bonded to a metal or a non-metal element
  • The metallic character of the element influences the acidic or basic behaviour of the molecule

Metals, non-metals & metalloids in Periodic Table, IGCSE & GCSE Chemistry revision notesMetals form basic oxides while non-metals form acidic oxides

 Acidic oxides

  • Acidic oxides are formed when a non-metal element combines with oxygen
  • They react with bases to form a salt and water
  • When dissolved in water they produce an acidic solution with a low pH
  • Common examples include CO2, SO2, NO2 and SiO2

Basic oxides

  • Basic oxides are formed when a metal element combines with oxygen
  • They react with acids to form a salt and water
  • When dissolved in water they produce a basic solution with a high pH
  • Common examples include CuO and CaO

Amphoteric Oxides

Amphoteric oxides

  • Amphoteric oxides are a curious group of oxides that can behave as both acidic and basic, depending on whether the other reactant is an acid or a base
  • In both cases, a salt and water are formed
  • Two of the most common amphoteric oxides are zinc oxide, ZnO and aluminum oxide, Al2O3
  • The hydroxides of both of these elements also behave amphoterically
  • Example of aluminium oxide behaving as a base:

Al2O3 + 6HCl → 2AlCl3 + 3H2O

  • Example of aluminium oxide behaving as an acid:

Al2O3 + 2NaOH → 2NaAlO2 + H2O

  • This acidic and basic behaviour is not easily explained by donating or accepting protons. A separate theory called the Lewis acid-base theory can identify acids or bases in these situations, but is not required for this course

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.