For nearly every substance on Earth, the solid form is denser than its liquid form and will sink when placed in it. An ice cube floating in a glass of water, therefore, represents a remarkable scientific oddity known as the density anomaly of water. This phenomenon is the result of forces acting at the molecular level, causing water to expand by about nine percent as it solidifies. This expansion means the same mass of water takes up more space as ice, making the solid less dense than its liquid counterpart.
Understanding Water’s Unique Bonds
The key to understanding this behavior lies in the molecular structure of water, a highly polar molecule. The oxygen atom attracts electrons, giving it a partial negative charge, while the two hydrogen atoms carry partial positive charges. This uneven charge distribution causes the positive end of one molecule to be drawn toward the negative end of a neighboring molecule.
These weak attractions are called hydrogen bonds, and they are constantly forming, breaking, and reforming within liquid water. This dynamic state allows the molecules to occasionally collapse into a relatively close-packed arrangement.
Liquid water achieves its maximum density not at the freezing point of 0°C, but slightly warmer, at 4°C. At this temperature, the molecules are energetic enough to pack together efficiently before the forces of attraction begin to organize them more rigidly.
The Transition to Solid Ice
As the temperature drops below 4°C and approaches the freezing point, the kinetic energy of the molecules decreases, allowing hydrogen bonds to exert a dominant influence. The bonds begin to lock into fixed positions, forcing the water molecules to arrange themselves into a precise, repeating geometric pattern.
The molecules organize into an open, three-dimensional structure known as a hexagonal crystal lattice (Ice I$\text{h}$). In this lattice, each oxygen atom is bonded to four hydrogen atoms, creating a network of repeating, six-sided rings.
This fixed, ordered structure is much less efficient at packing molecules than the liquid state. The geometry of the bonds dictates that the molecules must be held farther apart, introducing large pockets of empty space, or voids, into the solid structure. This increase in volume causes ice to be less dense than liquid water, allowing it to float.
Why This Matters for the Planet
The fact that ice floats has consequences for life on Earth, particularly in aquatic environments. When bodies of water cool in winter, the less dense ice forms a layer on the surface, acting as an insulating blanket. This protective layer shields the water beneath it from the frigid air, preventing the entire body of water from freezing solid.
Because the densest water sinks to the bottom, it usually remains at a stable 4°C, allowing aquatic organisms to survive the winter beneath the ice sheet. If ice behaved like most other solids and sank, bodies of water would freeze from the bottom up, eliminating most aquatic habitats.
The expansion of water as it freezes also plays a significant role in shaping the planet’s geology through freeze-thaw weathering. When liquid water seeps into cracks in rocks and then freezes, the nine percent increase in volume exerts immense pressure on the surrounding rock. Repeated cycles of freezing and thawing gradually widen the cracks and break down solid rock into smaller fragments.