Question

Describe the molecular basis of the property of water responsible for the presence of ice on the surface of a frozen lake.

Short answer

Hydrogen bonds create a lattice structure in ice, making it less dense than liquid water, causing it to float.

Step-by-step solution

- Understanding hydrogen bonding

Water molecules are held together by hydrogen bonds. Each water molecule can form up to four hydrogen bonds with neighboring water molecules due to its bent shape and the presence of two hydrogen atoms and one oxygen atom.

- Structure of ice

When water freezes, the molecules arrange themselves into a lattice structure. This structure is maintained by hydrogen bonds and creates a rigid and open hexagonal configuration.

- Density of ice vs. liquid water

The lattice structure of ice makes it less dense than liquid water. In the liquid state, water molecules are closer together due to the constant breaking and forming of hydrogen bonds. The open structure of ice means that it occupies more volume and thus has a lower density.

- Ice floating on water

Since ice is less dense than liquid water, it floats. This phenomenon is crucial for the formation of ice on the surface of lakes. As the top layer of water freezes, it floats and insulates the water below, preventing the entire lake from freezing solid.

Key concepts

Hydrogen Bonding

Water molecules are unique due to the presence of hydrogen bonds. These bonds occur because of the polar nature of water molecules.
Each water molecule consists of one oxygen atom and two hydrogen atoms. The oxygen atom is more electronegative, pulling electrons closer to itself.
This gives the oxygen atom a partial negative charge and the hydrogen atoms partial positive charges.
Due to these charges, each water molecule can form up to four hydrogen bonds with neighboring water molecules.
These bonds are not as strong as covalent bonds but are significant enough to influence the physical properties of water.

• In liquid water, hydrogen bonds are constantly breaking and reforming.
• In solid ice, hydrogen bonds form a stable lattice structure.

Understanding these hydrogen bonds is crucial to grasp the molecular basis of water's properties.

Structure of Ice

When water freezes, the molecules arrange themselves into a hexagonal lattice structure. This process is guided by the hydrogen bonds between the water molecules.
The hexagonal arrangement ensures that each water molecule maintains hydrogen bonds with four neighboring molecules.

This creates a rigid and open framework, significantly different from the more compact arrangement seen in liquid water. The hexagonal structure of ice is responsible for many of its unique properties:

• It makes ice less dense than liquid water.
• It gives ice its typically crystalline appearance.
• It results in an increased volume compared to the volume of water before freezing.

This lattice structure is what causes ice to float on water.

Density of Ice vs. Liquid Water

The density of a substance is determined by how much mass it has in a given volume. For water, density changes remarkably between its liquid and solid states.
In liquid water, the molecules are constantly moving and slipping past one another. Hydrogen bonds form and break frequently, which allows water molecules to pack closely.
This makes liquid water denser than solid ice.
When water freezes, the hydrogen bonds lock the water molecules into a structured, hexagonal lattice. This 'open' structure means there is more space between molecules in ice than in liquid water.

• Ice occupies more volume than an equal mass of liquid water.
• This results in ice having a lower density than liquid water.

This lower density means ice will float when placed in liquid water.

Ice Floating on Water

One of water’s most fascinating properties is that ice floats on liquid water. This occurs because ice is less dense than water due to its lattice structure.
As water starts to freeze, ice forms on the surface first, as the cooler, less dense ice floats above the warmer, denser liquid.
This has significant ecological implications:

• The ice layer insulates the water below, helping to protect aquatic life during cold periods.
• Prevents lakes and other bodies of water from freezing solid, preserving ecosystems.

Understanding why ice floats is essential for appreciating the balance of aquatic ecosystems and the role hydrogen bonding plays in this phenomenon.