Classification of Oxides

An oxide is a compound of oxygen with another element. We classify oxides by how they behave with acids and bases. This helps predict their reactions and links to whether the element is a metal or a non‑metal.

The four types

1) Basic oxides (usually metal oxides)

Basic oxides react with acids to form a salt and water (neutralisation). Examples: copper(II) oxide (CuO), calcium oxide (CaO).

Examples of reactions:

$$\text{CuO} + 2\text{HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{O}$$

$$\text{CaO} + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O}$$

2) Acidic oxides (usually non‑metal oxides)

Acidic oxides react with bases/alkalis to form a salt and water. Examples: sulfur dioxide (SO₂), carbon dioxide (CO₂).

Examples of reactions:

$$\text{CO}_2 + 2\text{NaOH} \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O}$$

$$\text{SO}_2 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_3 + \text{H}_2\text{O}$$

Some dissolve in water to give acids, for example: $$\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3$$

3) Amphoteric oxides

Amphoteric oxides react with acids and with bases. Examples: aluminium oxide (Al₂O₃), zinc oxide (ZnO).

With acid (acts like a base):

$$\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$$

With alkali (acts like an acid):

$$\text{Al}_2\text{O}_3 + 2\text{NaOH} \rightarrow 2\text{NaAlO}_2 + \text{H}_2\text{O}$$

4) Neutral oxides

Neutral oxides do not react with acids or bases. Examples: carbon monoxide (CO), nitrogen monoxide (NO).

Link to metallic and non‑metallic character

Most metal oxides are basic. Most non‑metal oxides are acidic. Some metals near the “borderline” (like aluminium and zinc) form amphoteric oxides. A few non‑metal oxides are neutral.

Real‑world connections

• Soil treatment: calcium oxide (basic) neutralises acidic soils.
• Blast furnace: basic CaO removes acidic SiO₂ to form slag (calcium silicate). $$\text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3$$
• Environment: SO₂ (acidic oxide) can cause acid rain; CO₂ makes rainwater slightly acidic.

Common misconceptions

• Basic oxide is not the same as alkali. Alkalis are bases that dissolve in water.
• Not all metal oxides are basic; Al₂O₃ and ZnO are amphoteric.
• Not all non‑metal oxides are acidic; CO and NO are neutral.