Elements react with oxygen (aka. burn in oxygen) in the atmosphere to form oxides.

Oxides are classified as:

  • Acidic
  • Basic
  • Amphoteric
  • Neutral


Acidic Oxides

Acidic oxides react with water to form acids.

  • Recall: Solutions of these acids in water have a pH value of less than 7.

Acidic oxides are oxides of non-metals, which is also known as non-metallic oxides. Examples:

  • $\text{Carbon} + \text{oxygen } \rightarrow \text{ carbon dioxide}$ $$\text{C} (\text{s}) + \text{O}_{2} (\text{g}) \rightarrow \text{CO}_{2} (\text{g})$$
  • $\text{Sulphur} + \text{oxygen } \rightarrow \text{ sulphur dioxide}$ $$\text{S} (\text{s}) + \text{O}_{2} (\text{g}) \rightarrow \text{SO}_{2} (\text{g})$$

Examples of acidic oxides are shown in the table below:

Acidic OxidesFormulaAcid produced with water
Sulphur Dioxide$\text{SO}_{2}$Sulphurous Acid


Sulphur Trioxide$\text{SO}_{3}$Sulphuric Acid


Carbon Dioxide$\text{CO}_{2}$Carbonic Acid


Phosphorus(V) Oxide$\text{P}_{4}\text{O}_{10}$Phosphoric Acid



Basic Oxides

Basic oxides react with acid to produce salt and water only.

  • Notice that this is a neutralization reaction.
  • Hence, basic oxides neutralize acids.

Basic oxides that are soluble in water are called alkalis.

  • Recall: Solutions of these alkalis in water have a pH value of more than 7.

Basic oxides are oxides of metals, which is also known as metallic oxides. Examples:

  • $\text{Sodium} + \text{oxygen } \rightarrow \text{ sodium oxide}$ $$\text{Na} (\text{s}) + \text{O}_{2} (\text{g}) \rightarrow 2\text{Na}_{2}\text{O} (\text{s})$$
  • $\text{Copper} + \text{oxygen } \rightarrow \text{ copper(II) oxide}$ $$\text{Cu} (\text{s}) + \text{O}_{2} (\text{g}) \rightarrow 2\text{CuO} (\text{s})$$

Examples of basic oxides are shown in the table below:

Basic OxidesFormula
Sodium Oxide$\text{Na}_{2}\text{O}$
Magnesium Oxide$\text{MgO}$
Calcium Oxide$\text{CaO}$
Copper Oxide$\text{CuO}$
Iron (III) Oxide$\text{Fe}_{2}\text{O}_{3}$


Amphoteric Oxides

Amphoteric oxides behave as an acidic oxide or as a basic oxide.

  • Amphoteric oxides show both acidic and basic properties.
  • Amphoteric oxides can react with both acids and alkalis to form salts and water. (Recall: Neutralization reaction)

Examples of amphoteric oxides are shown in the table below:

Amphoteric OxidesFormula
Aluminium Oxide$\text{Al}_{2}\text{O}_{3}$
Zinc Oxide$\text{ZnO}$
Lead(II) Oxide$\text{PbO}$

Reactions of amphoteric oxides with acids:

  • Aluminium oxide with hydrochloric acid $$\text{Al}_{2}\text{O}_{3} + 6 \text{HCl} \rightarrow 2\text{AlCl}_{3} + 3\text{H}_{2}\text{O}$$
  • Zinc oxide with sulphuric acid $$\text{ZnO} + \text{H}_{2}\text{SO}_{4} \rightarrow \text{ZnSO}_{4} + \text{H}_{2}\text{O}$$
  • Lead(II) oxide with nitric acid $$\text{PbO} + 2 \text{HNO}_{3} \rightarrow \text{Pb}\left( \text{NO}_{3} \right)_{2} + \text{H}_{2}\text{O}$$

Reactions of amphoteric oxides with bases:

  • Aluminium oxide with sodium hydroxide to form sodium aluminate $$\text{Al}_{2}\text{O}_{3} + 2 \text{NaOH} \rightarrow 2\text{NaAlO}_{2} + 3\text{H}_{2}\text{O}$$
  • Zinc oxide with sodium hydroxide to form sodium zincate $$\text{ZnO} + 2 \text{NaOH} \rightarrow \text{Na}_{2}\text{ZnO}_{2} + \text{H}_{2}\text{O}$$
  • Lead(II) oxide with sodium hydroxide to form sodium plumbate(II) $$\text{PbO} + 2 \text{NaOH} \rightarrow \text{Na}_{2}\text{PbO}_{2} + \text{H}_{2}\text{O}$$


Neutral Oxide

Neutral oxide has no acidic or basic properties. Examples:

  • Water
  • Carbon monoxide

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