Ice takes up more space than the liquid water from which it forms, a phenomenon unique among most substances. While nearly all other liquids contract and become denser when they solidify, water does the opposite. This volume increase means that ice is less dense than liquid water, which is why ice cubes and icebergs float. Understanding this difference requires examining the physical change and the underlying molecular structure.
Quantifying the Volume Difference
The volume difference between liquid water and solid ice is significant and easily measurable. When water freezes at standard atmospheric pressure, its volume increases by approximately 9% to 9.05%. This means that one liter of liquid water, when completely frozen, will occupy about 1.09 liters as ice.
This volume expansion directly translates to a decrease in density. Liquid water reaches its maximum density near 4°C, but upon freezing, the ice formed is about 9% less dense than the liquid water at the same temperature. For example, the density of ice is approximately 0.917 g/mL, compared to liquid water’s 1.00 g/mL at 0°C. This density difference is why ice floats, with only about 8% to 10% of an iceberg’s mass visible above the water line.
The Molecular Reason for Expansion
The unusual expansion of water upon freezing stems from the specific structure of its molecules, which are made of two hydrogen atoms and one oxygen atom in a bent shape. This geometric arrangement makes water a polar molecule, giving it a slight negative charge near the oxygen and a slight positive charge near the hydrogen atoms. These opposite partial charges allow water molecules to attract one another through weak electromagnetic forces called hydrogen bonds.
In liquid water, these hydrogen bonds constantly form, break, and reform as the molecules slide past each other, allowing them to pack relatively closely together. However, as the temperature drops toward the freezing point of 0°C, the molecules slow down, and the hydrogen bonds become more stable and fixed. This stabilization forces the molecules to arrange themselves into a highly organized, open crystalline structure known as a hexagonal lattice.
This fixed, honeycomb-like arrangement holds the water molecules farther apart than they were in the liquid state, creating open spaces or voids within the structure. The formation of this open lattice increases the overall volume of the water as it solidifies into ice. Since the molecules are locked into a pattern that is less compact than the liquid, this leads directly to the decrease in density and the increase in volume.
Practical Implications of Water’s Unique Property
The expansion of water upon freezing is a property that shapes both the Earth’s ecosystems and human engineering challenges. The fact that ice floats is ecologically significant, as it creates an insulating layer on the surface of lakes and rivers. This ice barrier protects the deeper liquid water below from the cold air, preventing entire bodies of water from freezing solid from the bottom up, which allows aquatic life to survive the winter.
The volume increase also generates immense force, impacting both built and natural environments. When water freezes inside a pipe, the 9% expansion exerts pressure that can exceed the pipe’s tensile strength, leading to bursting and property damage. This expansion is also a major driver of physical weathering, known as frost wedging. Water seeps into cracks in rocks and pavement, and when it freezes, the expanding ice acts like a powerful wedge, gradually breaking apart solid formations and creating potholes.