Classify oxides as acidic, including $mathrm{SO}_2$ and $mathrm{CO}_2$, or basic, including $mathrm{CuO}$ and $mathrm{CaO}$, related to metallic and non-metallic character

Acids, bases and salts – Oxides

Exam Tip: Remember that oxides of non‑metals are usually acidic, while oxides of metals are usually basic. Check the metallic character of the element to predict the oxide type.

What are oxides?

An oxide is a compound that contains oxygen bonded to another element. Think of it as oxygen giving a “handshake” to another element. The nature of that handshake (acidic or basic) depends on whether the other element is a metal or a non‑metal.

Classifying oxides

Acidic oxides (usually from non‑metals) react with water to form acids.

Basic oxides (usually from metals) react with water to form bases.

Examples

  • \$SO_2\$ (sulphur dioxide) – acidic oxide. Analogy: Like a sour lemon that turns water into a weak acid.

  • \$CO_2\$ (carbon dioxide) – acidic oxide. Analogy: Like a fizzy soda that dissolves in water to give a mild acid.

  • \$CuO\$ (copper(II) oxide) – basic oxide. Analogy: Like a strong soap that cleans and makes the solution basic.

  • \$CaO\$ (calcium oxide) – basic oxide. Analogy: Like a baking soda that makes the water alkaline.

Metallic vs Non‑metallic character

The position of an element on the periodic table tells us whether it behaves like a metal or a non‑metal. Elements on the left side (s‑block, p‑block non‑metals) tend to form acidic oxides, while those on the right side (d‑block metals) tend to form basic oxides.

OxideTypeReaction with WaterMetal/Non‑metal
\$SO_2\$Acidic\$SO2 + H2O \rightarrow H2SO3\$ (sulfurous acid)Non‑metal (S)
\$CO_2\$Acidic\$CO2 + H2O \rightarrow H2CO3\$ (carbonic acid)Non‑metal (C)
\$CuO\$Basic\$CuO + H2O \rightarrow Cu(OH)2\$ (copper(II) hydroxide)Metal (Cu)
\$CaO\$Basic\$CaO + H2O \rightarrow Ca(OH)2\$ (calcium hydroxide)Metal (Ca)

Exam Tip: When given a chemical formula, first check if the element is a metal or non‑metal. If it’s a metal, the oxide is likely basic; if it’s a non‑metal, the oxide is likely acidic. Use this rule to quickly answer classification questions.

Quick Review Questions

  1. Predict whether \$Fe2O3\$ is acidic or basic. Answer: Basic (iron is a metal).

  2. What happens when \$P4O{10}\$ reacts with water? Answer: Forms phosphoric acid (\$H3PO4\$), so it’s acidic.

  3. Why is \$Na_2O\$ basic? Answer: Sodium is a metal; its oxide reacts with water to give sodium hydroxide.

Exam Tip: For reaction equations, remember the general patterns:

  • Acidic oxide + \$H_2O \rightarrow\$ corresponding acid.

  • Basic oxide + \$H_2O \rightarrow\$ corresponding base.