Water shrinks on melting
Unlike most substances, liquid water is denser than solid ice.
Scientific Explanation
For most substances, the solid phase is denser than the liquid — molecules pack more tightly when they freeze. Water does the opposite: when ice melts, its volume decreases by about 9 percent. Liquid water is denser than ice, which is why ice floats.
The reason lies in the crystal structure of ice. In the ice crystal, each water molecule forms four hydrogen bonds with its neighbors, arranging itself in an open hexagonal lattice. This structure contains a significant amount of empty space, much like a framework of regular hexagons. When ice melts, this ordered network partially collapses, and the molecules can nestle closer together despite moving more freely. The result: less volume for the same mass, meaning a higher density.
Step by Step
Ice Crystal
In the ice crystal, each water molecule forms four hydrogen bonds with its neighbors. The molecules arrange themselves in an open hexagonal lattice -- with plenty of empty space between them.
Melting
As the temperature rises, the ordered network collapses. Hydrogen bonds break apart, and the molecules move closer together. They move more freely, yet pack more tightly.
Volume Comparison
The result: liquid water occupies about 9 percent less volume than ice. Same mass, less space -- density increases upon melting, not upon freezing.
The Anomaly
Most substances expand when they melt. Water does the opposite: it contracts. This is why ice floats on water -- and why lakes freeze from the top down.
Everyday Relevance
The fact that ice floats on water is one of the most familiar properties of water — and one of the most important for life on Earth. In winter, lakes and rivers freeze from the surface downward, while the water below remains liquid. This allows fish and other aquatic organisms to survive through the cold months.
Without this anomaly, bodies of water would freeze from the bottom up and could potentially freeze solid. Ice cubes in drinks, icebergs in the ocean, and the fact that water pipes can burst when water freezes inside them are all direct consequences of this remarkable property.