7.2 Oxides

2026 Syllabus Objectives

  1. Core: Classify oxides as acidic, including SO2\mathrm{SO}_2 and CO2\mathrm{CO}_2, or basic, including CuO\mathrm{CuO} and CaO\mathrm{CaO}, related to metallic and non-metallic character
  2. Supplement: Describe amphoteric oxides as oxides that react with acids and with bases to produce a salt and water
  3. Supplement: Classify Al2O3\mathrm{Al}_2\mathrm{O}_3 and ZnO\mathrm{ZnO} as amphoteric oxides

Classification of Oxides 🔬

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Oxides are chemical compounds formed when elements react with oxygen. The properties of these oxides depend on whether they are formed from metals or non-metals.

Non-Metal Oxides: Acidic Character

When non-metals burn in oxygen, they produce acidic oxides. These oxides have the following properties:

  • React with bases to form salts
  • Dissolve in water to produce acidic solutions
  • Turn blue litmus paper red

Key Definition:

Acidic oxides are oxides of non-metals that will react with bases and dissolve in water to produce acid solutions.

Common Examples of Acidic Oxides 📌

1. Sulfur Dioxide (SO2\mathrm{SO}_2)

When sulfur burns in oxygen, it produces sulfur dioxide gas:

S(s)+O2(g)→SO2(g)\mathrm{S}(s) + \mathrm{O}_2(g) \rightarrow \mathrm{SO}_2(g)
  • Burns with a bright blue flame
  • Produces a colourless gas
  • When water is added, it dissolves and turns litmus red

2. Carbon Dioxide (CO2\mathrm{CO}_2)

When carbon burns in oxygen, it produces carbon dioxide gas:

C(s)+O2(g)→CO2(g)\mathrm{C}(s) + \mathrm{O}_2(g) \rightarrow \mathrm{CO}_2(g)
  • Carbon glows red when burning
  • Produces a colourless gas
  • Dissolves slightly in water and slowly turns litmus red

3. Phosphorus(V) Oxide (P2O5\mathrm{P}_2\mathrm{O}_5)

  • Phosphorus burns with a yellow flame
  • Produces a white solid
  • Dissolves and turns litmus red

Metal Oxides: Basic Character

When metals burn in oxygen, they produce basic oxides. These oxides have the following properties:

  • React with acids to neutralize them
  • If soluble, dissolve in water to produce alkaline solutions
  • Turn red litmus paper blue

Key Definition:

Basic oxides are oxides of metals that will react with acids to neutralise the acid.

Common Examples of Basic Oxides 📌

1. Calcium Oxide (CaO\mathrm{CaO})

  • Calcium burns with a red flame
  • Produces a white solid
  • Dissolves in water and turns litmus blue

2. Copper Oxide (CuO\mathrm{CuO})

  • Copper does not burn, but turns black when heated
  • Produces a black solid
  • Insoluble in water

3. Magnesium Oxide (MgO\mathrm{MgO})

  • Magnesium burns with a bright white flame
  • Produces a white solid
  • Dissolves slightly and turns litmus blue

4. Sodium Oxide (Na2O\mathrm{Na}_2\mathrm{O})

  • Sodium burns with a yellow flame
  • Produces a white solid
  • Dissolves and turns litmus blue

5. Iron Oxide (FeO\mathrm{FeO})

  • Iron burns with yellow sparks
  • Produces a blue-black solid
  • Insoluble in water

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