Question

In most compounds, the solid phase is denser than the liquid phase. Why isn't this true for water?

Short answer

In conclusion, the reason why water has a solid phase (ice) with a lower density than its liquid phase is due to the distinct hexagonal lattice structure formed by water molecules when they freeze, which is a direct result of hydrogen bonding between the polar water molecules.

Step-by-step solution

Understand the structure of a water molecule

A water molecule (H2O) has a bent structure with an oxygen atom in the center and two hydrogen atoms bonded to it. The atoms are bonded through covalent bonds, and since oxygen is more electronegative than hydrogen, there's a slight charge difference between them, resulting in a polar molecule.

The hydrogen bonding in water

Due to the polar nature of the water molecules, they form hydrogen bonds with neighboring water molecules. An oxygen atom in a water molecule attracts the positively charged hydrogen atoms from the neighboring water molecules, forming the hydrogen bonds. These bonds significantly affect the properties of water, including the density difference between the solid and liquid phases.

The structure of water in the liquid phase

In the liquid phase, water molecules are constantly moving and forming and breaking hydrogen bonds with the neighboring water molecules. This constant motion allows the water molecules to be relatively close together, resulting in a higher density.

The structure of water in the solid phase (ice)

When water freezes and forms ice, the water molecules slow down and arrange themselves in a hexagonal lattice structure. In this structure, each water molecule forms hydrogen bonds with four neighboring water molecules. As a result, the water molecules are held at a fixed distance from each other, creating open spaces within the hexagonal lattice. This regular arrangement of water molecules with open spaces results in a solid phase with a lower density than the liquid phase.

Comparing the density of solid and liquid water

In most substances, the solid phase has a higher density than the liquid phase due to more closely-packed molecules. However, in the case of water, the hydrogen bonds lead to a unique lattice structure in the solid phase, with open spaces that make ice less dense than liquid water. In conclusion, the reason why water has a solid phase (ice) with a lower density than its liquid phase is due to the distinct hexagonal lattice structure formed by water molecules when they freeze, which is a direct result of hydrogen bonding between the polar water molecules.

Key concepts

Water Molecule Structure

A water molecule, represented by the chemical formula H extsubscript{2}O, is composed of one oxygen atom and two hydrogen atoms. These atoms form covalent bonds, meaning they share electrons. The structure is not linear but bent, giving it a unique, angular shape. This occurs because oxygen, being highly electronegative, pulls the shared electrons closer to itself. The result is a molecule with a distinct polarity. Oxygen ends up with a slight negative charge, while the hydrogen atoms gain a slight positive charge.
This polarity is key to understanding many of water’s unique properties, especially its ability to form hydrogen bonds. Due to its bent shape and polar nature, the water molecule can interact strongly with other water molecules, as well as with other substances, through these hydrogen bonds. Such bonding is foundational to the behaviors of water discussed in further sections.

Density Anomaly of Water

One of water’s most fascinating properties is how its density behaves unusually as it transitions from liquid to solid. In most substances, solid phases are denser than liquid phases. However, water defies this norm, primarily due to the effects of hydrogen bonding.
In its liquid state, water molecules continuously move, forming and breaking hydrogen bonds with neighboring molecules. This constant motion allows them to pack closely together, resulting in a high density. However, when water cools and freezes, the movement of molecules slows down significantly. The hydrogen bonds then force the molecules into a fixed, open framework, which increases the space between them.

• This spacing causes the solid form of water, ice, to be less dense than its liquid form.
• As a consequence, ice floats on liquid water, an essential feature for aquatic life in cold climates.

Understanding this density anomaly helps in explaining many of water's essential environmental roles.

Hexagonal Lattice Ice Structure

When water transitions to ice, the molecular arrangement changes drastically. The decrease in temperature allows for stabilization of hydrogen bonds, and water molecules lock into a specific pattern. This pattern is a hexagonal lattice structure, where each water molecule bonds with four other neighboring molecules. The rigid hexagonal formation has inherent open spaces between the molecules.
The hexagonal lattice accounts for several of ice's unique characteristics:

• Its lower density than liquid water, resulting in ice floating.
• Its capacity to insulate the water below from colder air temperatures, providing a habitat for life even in frozen environments.

This structure underpins many of the phenomena observed in nature and is crucial for the survival of ecosystems that rely on water's insulation properties.