7.5 Oxides

Introduction

Binary compounds of elements with oxygen are called oxides.

For example: CaO, CO₂, SO₂, Na₂O, CuO, Al₂O₃, etc.

Oxides can be classified as acidic, basic, or amphoteric based on their chemical properties and their relationship with metallic and non-metallic elements.

This section explains the three types of oxides and provides examples of each.

Definition

Oxides: Binary compounds of elements with oxygen are called oxides.

In simple words: An oxide is a compound made of oxygen and one other element.

Main Idea

Oxides can be classified into three types based on their chemical properties:

• Acidic Oxides — form acids when they react with water; they are oxides of non-metals.
• Basic Oxides — form bases when they react with water; they are oxides of metals.
• Amphoteric Oxides — can react both with acids and bases, displaying both acidic and basic properties; they are oxides of some metals.

Core Concept

Key point: The classification of an oxide depends on whether it reacts with water to form an acid or a base, or whether it can react with both acids and bases.

Detailed Explanation

1. Acidic Oxides

What they are: Acidic oxides typically form acids when they react with water.

What they are made of: They are oxides of non-metals.

Examples:

SO₂ (Sulphur dioxide): Reacts with water to form sulphurous acid (H₂SO₃).

SO_2(g) + H₂O_(l) → H₂SO_3(aq)

CO₂ (Carbon dioxide): Reacts with water to form carbonic acid (H₂CO₃).

CO_2(g) + H₂O_(l) → H₂CO_3(aq)

Key point: Acidic oxides are non-metal oxides that form acids when they react with water.

2. Basic Oxides

What they are: Basic oxides form bases when they react with water.

What they are made of: They are oxides of metals.

Examples:

Na₂O (Sodium oxide): Reacts with acids to form salts and water, indicating its basic nature.

Na₂O_(s) + 2HCl_(aq) → 2NaCl_(aq) + H₂O_(l)

CaO (Calcium Oxide): Reacts with water to form calcium hydroxide (Ca(OH)₂), a strong base. It reacts with acids to form salts.

CaO_(s) + H₂O_(l) → Ca(OH)_2(aq)

CaO_(s) + 2HCl_(aq) → CaCl_2(aq) + H₂O_(l)

Key point: Basic oxides are metal oxides that form bases when they react with water.

3. Amphoteric Oxides

What they are: Amphoteric oxides can react both with acids and bases, displaying both acidic and basic properties.

What they are made of: These are oxides of some metals.

Examples:

Al₂O₃ (Aluminum Oxide): Reacts with acids to form salts and water, and with bases to form aluminates.

With acid (HCl): Al₂O_3(s) + 6HCl_(aq) → 2AlCl_3(aq) + 3H₂O_(l)

With base (NaOH): Al₂O_3(s) + 2NaOH_(aq) + 3H₂O_(l) → 2NaAl(OH)_4(aq)

Product: Sodium aluminate

ZnO (Zinc Oxide): Reacts with acids to form salts and water, and with bases to form zincates.

With acid (HCl): ZnO_(s) + 2HCl_(aq) → ZnCl_2(aq) + H₂O_(l)

With base (NaOH): ZnO_(s) + 2NaOH_(aq) + H₂O_(l) → Na₂[Zn(OH)₄]_(aq)

Product: Sodium zincate

Key point: Amphoteric oxides are metal oxides that can react with both acids and bases.

Summary Table

Intermediate Level

Acidic oxides:

• Non-metal oxides.
• React with water to form acids.
• Examples: SO₂ → H₂SO₃; CO₂ → H₂CO₃.

Basic oxides:

• Metal oxides.
• React with water to form bases.
• Examples: Na₂O + HCl → NaCl + H₂O; CaO + H₂O → Ca(OH)₂.

Amphoteric oxides:

• Some metal oxides.
• React with both acids and bases.
• Examples: Al₂O₃ + HCl → AlCl₃ + H₂O; Al₂O₃ + NaOH → sodium aluminate; ZnO + HCl → ZnCl₂ + H₂O; ZnO + NaOH → sodium zincate.

Key point: The classification of oxides is based on their chemical properties and their relationship with metallic and non-metallic elements.

Solved Example

Example: Classify the Following Oxides

Question: Classify the following oxides as acidic, basic, or amphoteric:

• SO₂
• CaO
• Al₂O₃
• CO₂
• Na₂O
• ZnO

Solution:

• SO₂ → Acidic oxide (non-metal oxide, forms sulphurous acid with water)
• CaO → Basic oxide (metal oxide, forms calcium hydroxide with water)
• Al₂O₃ → Amphoteric oxide (reacts with both acids and bases)
• CO₂ → Acidic oxide (non-metal oxide, forms carbonic acid with water)
• Na₂O → Basic oxide (metal oxide, reacts with acids to form salts and water)
• ZnO → Amphoteric oxide (reacts with both acids and bases)

Final Answer:

• Acidic oxides: SO₂, CO₂
• Basic oxides: CaO, Na₂O
• Amphoteric oxides: Al₂O₃, ZnO

Important Points

• - Oxides = binary compounds of elements with oxygen.
• - Acidic oxides = non-metal oxides that form acids with water.
• - Examples: SO₂, CO₂
• - Basic oxides = metal oxides that form bases with water.
• - Examples: Na₂O, CaO
• - Amphoteric oxides = some metal oxides that react with both acids and bases.
• - Examples: Al₂O₃, ZnO
• - Acidic oxides typically form acids when they react with water.
• - Basic oxides form bases when they react with water.
• - Amphoteric oxides can react both with acids and bases, displaying both acidic and basic properties.

Remember

• - Oxides = binary compounds with oxygen.
• - Acidic oxides = non-metal oxides, form acids with water.
• - Basic oxides = metal oxides, form bases with water.
• - Amphoteric oxides = some metal oxides, react with both acids and bases.
• - Examples:
• - Acidic: SO₂, CO₂
• - Basic: Na₂O, CaO
• - Amphoteric: Al₂O₃, ZnO
• - SO₂ + H₂O → H₂SO₃.
• - CO₂ + H₂O → H₂CO₃.
• - CaO + H₂O → Ca(OH)₂.
• - Al₂O₃ + HCl → AlCl₃ + H₂O.
• - Al₂O₃ + NaOH → sodium aluminate.
• - ZnO + HCl → ZnCl₂ + H₂O.
• - ZnO + NaOH → sodium zincate.

Common Mistakes

Quick Revision

Oxides:

Binary compounds of elements with oxygen are called oxides. For example, CaO, CO₂, SO₂, Na₂O, CuO, Al₂O₃ etc.

Oxides can be classified as acidic, basic, or amphoteric based on their chemical properties and their relationship with metallic and non-metallic elements.

• 1. Acidic Oxides:
• Acidic oxides typically form acids when they react with water. They are oxides of non-metals. Examples include:

• SO₂ (Sulphur dioxide): Reacts with water to form sulphurous acid H₂SO₃.
• CO₂ (Carbon Dioxide): Reacts with water to form carbonic acid (H₂CO₃).

• 2. Basic Oxides:
• Basic oxides form bases when they react with water. They are oxides of metals. Examples include:

• - Na₂O (sodium oxide): Reacts with acids to form salts and water, indicating its basic nature.
• Na₂O_(s) + 2HCl_(aq) → 2NaCl_(aq) + H₂O_(l)
• - CaO (Calcium Oxide): Reacts with water to form calcium hydroxide (Ca(OH)₂), a strong base. It reacts with acids to form salt.
• CaO_(s) + H₂O_(l) → Ca(OH)_2(aq)
• CaO_(s) + 2HCl_(aq) → CaCl_2(aq) + H₂O_(l)

• 3. Amphoteric Oxides:
• Amphoteric oxides can react both with acids and bases, displaying both acidic and basic properties. These are oxides of some metals. Examples include:

• - Al₂O₃ (Aluminum Oxide): Reacts with acids to form salts and water, and with bases to form aluminates.
• - With acid (HCl):
• Al₂O_3(s) + 6HCl_(aq) → 2AlCl_3(aq) + 3H₂O_(l)
• - With base (NaOH):
• Al₂O_3(s) + 2NaOH_(aq) + 3H₂O_(l) → 2NaAl(OH)_4(aq)
• - Product: Sodium aluminate

• - ZnO (Zinc Oxide): Reacts with acids to form salts and water, and with bases to form zincates.
• - With acid (HCl):
• ZnO_(s) + 2HCl_(aq) → ZnCl_2(aq) + H₂O_(l)
• - With base (NaOH):
• ZnO_(s) + 2NaOH_(aq) + H₂O_(l) → Na₂[Zn(OH)₄]_(aq)
• - Product: Sodium zincate

60-Second Revision

• Oxides = binary compounds with oxygen.
• Acidic oxides = non-metal oxides, form acids with water.
• Basic oxides = metal oxides, form bases with water.
• Amphoteric oxides = some metal oxides, react with both acids and bases.
• Examples:
• Acidic: SO₂, CO₂
• Basic: Na₂O, CaO
• Amphoteric: Al₂O₃, ZnO
• SO₂ + H₂O → H₂SO₃.
• CO₂ + H₂O → H₂CO₃.
• CaO + H₂O → Ca(OH)₂.
• Al₂O₃ + HCl → AlCl₃ + H₂O.
• Al₂O₃ + NaOH → sodium aluminate.
• ZnO + HCl → ZnCl₂ + H₂O.
• ZnO + NaOH → sodium zincate.
• ⚠️ Warning: not all metal oxides are basic — some are amphoteric (e.g., Al₂O₃, ZnO).