10.5 Oxides and Hydroxides

This topic explains metal oxides and metal hydroxides — how they form, their reactions with water, and the concept of amphoteric oxides and hydroxides.

Definition

Metal Oxides

Metal oxides are compounds formed by the reaction of metals with oxygen.

Simple explanation: Metal oxides are what you get when a metal reacts with oxygen.

Metal Hydroxides

Metal hydroxides are compounds formed when metal oxides dissolve in water.

Simple explanation: Metal hydroxides form when metal oxides react with water.

Amphoteric Oxides and Hydroxides

Amphoteric oxides and hydroxides are metal oxides and hydroxides that do not dissolve in water but behave in chemical reactions like both acids and bases.

Simple explanation: Amphoteric oxides and hydroxides can react with both acids and bases.

Main Idea

Bases and hydroxides of metals consist of oxides and hydroxides of metals.

• Metallic oxides are formed by the reaction of metals with oxygen.
• When metal oxides dissolve in water, they form metal hydroxides.
• Most metal oxides and hydroxides are basic in nature.
• Some metal oxides and hydroxides are amphoteric — they react with both acids and bases.

Simple explanation: Metal oxides and hydroxides are usually basic, but some are amphoteric (react with both acids and bases).

Core Concept

Formation of Metal Oxides

General equation:

Metal + Oxygen → Metal oxide

Examples from the book:

Sodium reacts with oxygen:

4Na + O₂ → 2Na₂O

Magnesium burns in air:

2Mg + O₂ → 2MgO

Simple explanation: Metals react with oxygen to form metal oxides.

Formation of Metal Hydroxides

When metal oxides dissolve in water, they form metal hydroxides.

General equation:

Metal oxide + Water → Metal hydroxide

Examples from the book:

Magnesium oxide reacts with water:

MgO + H₂O → Mg(OH)₂

Sodium oxide reacts with water:

Na₂O + H₂O → 2NaOH

Calcium oxide reacts with water:

CaO + H₂O → Ca(OH)₂

Simple explanation: Metal oxides react with water to form metal hydroxides.

Detailed Explanation

Why Metal Oxides Are Basic

• They produce metal cations and oxide ions in aqueous solution.
• Because oxide ions are unstable in water, they immediately accept protons from water molecules and become hydroxide ions.
• The water molecules also become hydroxide ions.

Reaction:

O^2- + H–OH → 2OH^-

Simple explanation: Oxide ions grab protons from water, producing hydroxide ions — which makes the solution basic.

Which Species Is the Proton Donor?

In the reaction:

O^2- + H–OH → 2OH^-

• Water (H–OH) is the proton donor (acid).
• Oxide ion (O²⁻) is the proton acceptor (base).

Simple explanation: Water gives a proton to the oxide ion, forming two hydroxide ions.

Most Metal Oxides and Hydroxides Are Basic

Most metal oxides and hydroxides are very basic in nature. They show the characteristic properties of bases.

• Na₂O → basic
• MgO → basic
• CaO → basic
• NaOH → basic
• Mg(OH)₂ → basic
• Ca(OH)₂ → basic

Simple explanation: Most metal oxides and hydroxides behave like bases.

Amphoteric Oxides and Hydroxides

However, some metal oxides and hydroxides do not dissolve in water but behave in chemical reactions like both acids and bases. Such metal oxides and hydroxides are called amphoteric oxides and hydroxides.

• Aluminium oxide (Al₂O₃)
• Aluminium hydroxide (Al(OH)₃)
• Zinc oxide (ZnO)
• Zinc hydroxide (Zn(OH)₂)

Simple explanation: Amphoteric oxides and hydroxides are "two-faced" — they can react with both acids and bases.

How Amphoteric Oxides React

With acid (behaving as a base):

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

With base (behaving as an acid):

Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O

Simple explanation: Amphoteric oxides can neutralize both acids and bases.

Summary Table

Intermediate Level

Why Metal Oxides Are Basic

• They contain oxide ions (O²⁻).
• When dissolved in water, O²⁻ accepts protons from water:

O^2- + H–OH → 2OH^-

3. This produces OH⁻ ions, which make the solution basic.

Simple explanation: Oxide ions grab protons from water, producing hydroxide ions — which makes the solution basic.

Why Some Oxides Are Amphoteric

• React with acids — behaving as bases.
• React with bases — behaving as acids.

• Accept protons (like a base).
• Donate protons (like an acid).

Simple explanation: Amphoteric oxides can "switch roles" depending on what they react with.

Examples of Amphoteric Reactions

Aluminium oxide with acid (behaving as a base):

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

Aluminium oxide with base (behaving as an acid):

Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O

Simple explanation: Amphoteric oxides react with both acids and bases, forming different products.

Applications of Metal Oxides and Hydroxides

Simple explanation: Metal oxides and hydroxides are widely used in industry and daily life.

Comparison of Basic and Amphoteric Oxides

Important Points

• - Metal oxides: formed by reaction of metals with oxygen.
• - Metal hydroxides: formed when metal oxides dissolve in water.
• - Most metal oxides and hydroxides are basic.
• - Amphoteric oxides and hydroxides: do not dissolve in water but react with both acids and bases.
• - Examples of amphoteric oxides/hydroxides: Al₂O₃, Al(OH)₃, ZnO, Zn(OH)₂.
• - Equations:
• - Metal + Oxygen → Metal oxide
• - Metal oxide + Water → Metal hydroxide
• - O²⁻ + H–OH → 2OH⁻
• - Examples:
• - 4Na + O₂ → 2Na₂O
• - 2Mg + O₂ → 2MgO
• - MgO + H₂O → Mg(OH)₂
• - Na₂O + H₂O → 2NaOH
• - CaO + H₂O → Ca(OH)₂

Remember

• - Metal + Oxygen → Metal oxide.
• - Metal oxide + Water → Metal hydroxide.
• - Most metal oxides/hydroxides are basic.
• - Amphoteric oxides/hydroxides react with both acids and bases.
• - Examples of amphoteric: Al₂O₃, Al(OH)₃, ZnO, Zn(OH)₂.
• - Equations:
• - 4Na + O₂ → 2Na₂O
• - 2Mg + O₂ → 2MgO
• - MgO + H₂O → Mg(OH)₂
• - Na₂O + H₂O → 2NaOH
• - CaO + H₂O → Ca(OH)₂

Common Mistakes

• 1. Thinking all metal oxides dissolve in water. Some do not (like Al₂O₃).
• 2. Thinking all metal oxides are basic. Some are amphoteric (Al₂O₃, ZnO).
• 3. Confusing basic and amphoteric oxides. Basic oxides react only with acids; amphoteric react with both acids and bases.
• 4. Forgetting that metal hydroxides form from metal oxides + water. This is the key reaction.
• 5. Confusing metal oxides with non-metal oxides. Non-metal oxides (like CO₂, SO₂) are usually acidic.
• 6. Thinking amphoteric means neutral. Amphoteric means it can act as both acid and base.

Quick Revision

• Metal oxides: compounds formed by the reaction of metals with oxygen.
• Metal + Oxygen → Metal oxide
• 4Na + O₂ → 2Na₂O
• 2Mg + O₂ → 2MgO
• Metal hydroxides: formed when metal oxides dissolve in water.
• Metal oxide + Water → Metal hydroxide
• MgO + H₂O → Mg(OH)₂
• Na₂O + H₂O → 2NaOH
• CaO + H₂O → Ca(OH)₂
• Most metal oxides and hydroxides are basic in nature.
• Amphoteric oxides and hydroxides: do not dissolve in water but behave in chemical reactions like both acids and bases.
• Examples: Al₂O₃, Al(OH)₃, ZnO, Zn(OH)₂
• Reaction of oxide ions with water:
• O²⁻ + H–OH → 2OH⁻
• Water is the proton donor (acid); oxide ion is the proton acceptor (base).

60-Second Revision

• Metal + Oxygen → Metal oxide.
• Metal oxide + Water → Metal hydroxide.
• Most metal oxides/hydroxides are basic.
• Amphoteric: Al₂O₃, ZnO — react with both acids and bases.
• Equations: 2Mg + O₂ → 2MgO; MgO + H₂O → Mg(OH)₂.
• Warning: not all metal oxides dissolve in water — some are amphoteric.