3.9.2 Peculiar Behaviour of Hydrogen Bonding

This topic explains the unusual behaviour of water due to hydrogen bonding — specifically, why ice is less dense than liquid water.

Definition

Peculiar Behaviour of Hydrogen Bonding: The unusual physical properties of water that arise due to the presence of hydrogen bonding between water molecules.

Main Idea

Most solids are denser than their liquids. However, ice is less dense than water. This is because in ice, there is a three-dimensional hydrogen-bonded network of water molecules. This produces a rigid lattice in which each oxygen atom is surrounded by a tetrahedron of hydrogen atoms. This "more open" arrangement, due to the relatively long hydrogen bonds, allows the water molecules to be slightly farther apart than in the liquid. Hence water in the solid state occupies more space. So, the density of ice is less than that of liquid water.

Core Concept

Hydrogen bonding in water creates a tetrahedral network in ice. This open structure has empty spaces between the molecules, making ice less dense than liquid water. That is why ice floats on water.

Detailed Explanation

Ice is Less Dense Than Water

• Most of the solids are denser than their liquids.
• This is because, in the solid state, the molecules are more closely packed.
• But this is not true of water.
• In ice, there is a three-dimensional hydrogen-bonded network of water molecules.
• This produces a rigid lattice in which each oxygen atom is surrounded by a tetrahedron of hydrogen atoms.
• This "more open" arrangement, due to the relatively long hydrogen bonds, allows the water molecules to be slightly farther apart than in the liquid.
• Hence, water in the solid state occupies more space.
• So, the density of ice is less than that of liquid water.
• That is why ice floats on water.

The Hydrogen-Bonded Network in Ice

• Each water molecule can form four hydrogen bonds.
• Two hydrogen bonds are through the hydrogen atoms (donating).
• Two hydrogen bonds are through the lone pairs on oxygen (accepting).
• This creates a tetrahedral arrangement in ice.
• The tetrahedral arrangement has empty spaces between the molecules.
• These empty spaces make ice less dense than water.

Why This is Peculiar

• Normally, cooling a liquid makes it contract and become denser.
• Water behaves differently:
• When water cools, it contracts until 4°C.
• Below 4°C, water expands as it forms ice.
• Ice is less dense than liquid water.
• This is a peculiar (anomalous) property of water.

The Strength of Intermolecular Forces

The strength of intermolecular forces follows this order:

H-bonding > permanent dipole-dipole > instantaneous dipole-induced dipole forces

Comparison of Boiling Points

Key Point

• Melting and boiling points of covalent molecules depend upon the intermolecular forces, not on the covalent bonds in the molecule.
• Covalent bonds are very strong (values in hundreds of kJ mol⁻¹).
• The forces between molecules are much weaker than covalent bonds.

Intermediate Level

Ice is Less Dense Than Water

• Most solids are denser than their liquids.
• Water is an exception: ice is less dense than liquid water.
• Reason: In ice, water molecules form a three-dimensional hydrogen-bonded network.
• This produces a rigid lattice with each oxygen atom surrounded by a tetrahedron of hydrogen atoms.
• This open arrangement has empty spaces.
• The water molecules are farther apart in ice than in liquid water.
• Hence, ice is less dense than water.
• That is why ice floats on water.

The Hydrogen-Bonded Network in Ice

• Each water molecule forms four hydrogen bonds.
• Two through hydrogen atoms (donating).
• Two through lone pairs on oxygen (accepting).
• This creates a tetrahedral arrangement.
• The tetrahedral arrangement has empty spaces.

Comparison of Boiling Points

Strength Order

H-bonding > Dipole-dipole > London forces

Important Points

• 1. Ice is less dense than liquid water.
• 2. Reason: Three-dimensional hydrogen-bonded network in ice.
• 3. This network has empty spaces.
• 4. Water molecules are farther apart in ice.
• 5. Ice floats on water.
• 6. This is a peculiar (anomalous) property of water.
• 7. Strength: H-bonding > Dipole-dipole > London forces.
• 8. MP/BP depend on intermolecular forces, not covalent bonds.

Remember

• - Ice is less dense than water
• - Reason: Hydrogen-bonded network with empty spaces
• - Result: Ice floats on water
• - Strength: H-bonding > Dipole-dipole > London
• - MP/BP: Depend on intermolecular forces

Common Mistakes

• 1. Thinking all solids are denser than their liquids: Ice is an exception.
• 2. Forgetting the role of hydrogen bonding: It creates the open structure in ice.
• 3. Thinking hydrogen bonding is a covalent bond: It is an intermolecular force.
• 4. Ignoring the importance of ice floating: It protects aquatic life.

Quick Revision

• Ice is less dense than liquid water.
• Reason: Three-dimensional hydrogen-bonded network in ice.
• This network has empty spaces.
• Water molecules are farther apart in ice.
• Ice floats on water.
• This is a peculiar (anomalous) property of water.
• Strength: H-bonding > Dipole-dipole > London forces.
• MP/BP depend on intermolecular forces, not covalent bonds.

60-Second Revision

• Ice is less dense than water
• Reason: Hydrogen-bonded network with empty spaces
• Result: Ice floats on water
• Strength: H-bonding > Dipole-dipole > London
• MP/BP: Depend on intermolecular forces

QUALITY VALIDATION

• Topic: 3.9.2 Peculiar Behaviour of Hydrogen Bonding
• Chapter: 3 (Chemical Bonding)

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