Amphoteric oxides are oxides that exhibit both acidic and basic properties. This means they can react with both acids and bases to form a salt and water. The term 'amphoteric' literally means 'having two different forms' (from the Greek 'amphos' meaning both and 'terikos' meaning form). In the context of oxides, it refers to their ability to react with both acidic and basic reagents.

Two examples of amphoteric oxides are aluminium oxide (Al₂O₃) and zinc oxide (ZnO).

Reaction with Hydrochloric Acid (HCl):

• Aluminium oxide reacts with hydrochloric acid to form aluminium chloride and water. The aluminium ion (Al³⁺) reacts with the hydrogen ions (H⁺) from the acid.
• Zinc oxide reacts with hydrochloric acid to form zinc chloride and water. The zinc ion (Zn²⁺) reacts with the hydrogen ions (H⁺) from the acid.

Balanced chemical equations:

1. Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)
2. ZnO(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l)

a Sulfur dioxide (SO₂) is acidic. This is because sulfur is a non-metal and oxygen is a non-metal. The electronegativity difference between sulfur and oxygen is relatively small, so the bond is not highly polar. When SO₂ dissolves in water, it reacts with water molecules to form sulfurous acid (H₂SO₃). The resulting sulfurous acid donates protons (H⁺) to water, increasing the concentration of H⁺ ions in solution, which is characteristic of an acidic solution. The non-metallic character of both sulfur and oxygen contributes to the acidic nature of the oxide.

b Copper(II) oxide (CuO) is basic. Copper is a metal and oxygen is a non-metal. The electronegativity difference between copper and oxygen is significant, leading to a polar covalent bond with a partial negative charge on the oxygen and a partial positive charge on the copper. When CuO dissolves in water, the oxygen atoms attract the hydrogen ions (H⁺) from water molecules. This results in an increase in the concentration of hydroxide ions (OH^-) in solution, which is characteristic of a basic solution. The metallic character of copper and the non-metallic character of oxygen contribute to the basic nature of the oxide.

Generally, oxides of metals tend to be basic, while oxides of non-metals tend to be acidic. This relationship stems from the electronegativity difference between the element and oxygen. Metals have low electronegativity and tend to lose electrons to form positive ions. When a metal reacts with oxygen, it forms a positive oxide ion (Mⁿ⁺). The resulting Mⁿ⁺ ion has a strong attraction for the negatively charged oxygen atoms in water, leading to the formation of hydroxide ions (OH^-) and a basic solution.

Non-metals have high electronegativity and tend to gain electrons to form negative ions. When a non-metal reacts with oxygen, it forms a negative oxide ion (X^-). The resulting X^- ion has a weak attraction for the positively charged hydrogen ions (H⁺) in water, leading to the formation of hydronium ions (H₃O⁺) and an acidic solution.

Examples:

• Sodium oxide (Na₂O) is basic. Sodium is a metal with low electronegativity. Na₂O reacts with water to form sodium hydroxide (NaOH), a strong base.
• Carbon dioxide (CO₂) is acidic. Carbon is a non-metal with high electronegativity. CO₂ reacts with water to form carbonic acid (H₂CO₃), which donates protons (H⁺) to water, resulting in an acidic solution.