Yes, water expands by about 9% in volume when it freezes. This makes ice less dense than liquid water, which is why ice floats. It’s also why a forgotten bottle of water will crack in your freezer and why potholes appear every spring. This expansion is one of water’s most unusual properties, and very few other substances behave this way.
Why Water Expands Instead of Shrinking
Most liquids contract when they solidify. Their molecules pack closer together in a solid state, making the solid denser than the liquid. Water does the opposite, and the reason comes down to how water molecules connect to each other.
In liquid water, molecules are constantly moving and tumbling past one another. They form temporary connections called hydrogen bonds, but those bonds break and reform millions of times per second. The molecules stay relatively close together, with no fixed arrangement.
When water freezes, those hydrogen bonds lock into place permanently. Each water molecule bonds to four neighbors in a rigid, tetrahedral shape, with angles of about 109 degrees between connections. This creates a hexagonal crystal structure, the same six-sided symmetry visible in snowflakes. The critical detail is that this arrangement is “open,” meaning it has gaps and empty space built into the structure. Those gaps are why ice takes up more room than the liquid water it came from. Ice has a density of about 0.917 g/cm³, compared to liquid water’s 1.000 g/cm³ at 4°C.
The Strange Behavior Starts Before Freezing
Water actually reaches its maximum density at 4°C, not at 0°C. As water cools below 4°C, it starts becoming less dense even while still liquid. This happens because clusters of molecules begin forming those semi-rigid hydrogen bond networks before full freezing occurs, gradually pushing molecules slightly farther apart.
This is why lakes freeze from the top down. As surface water cools below 4°C, it becomes lighter than the warmer water beneath it and stays on top. Once it hits 0°C, it freezes into a floating layer of ice. The denser, slightly warmer water sinks to the bottom, which is how fish and other aquatic life survive winter under an insulating cap of ice.
The Force Behind Burst Pipes
A 9% volume increase sounds modest, but in a confined space, the pressure is enormous. Freezing water can generate up to roughly 43,500 pounds per square inch (psi) of force. For comparison, a standard copper water pipe in your home is designed to handle about 1,500 psi. That’s less than 4% of what expanding ice can deliver, which is why frozen pipes don’t just crack. They rupture.
The burst doesn’t always happen at the spot where ice forms. As ice expands inside a pipe, it compresses the liquid water still trapped between the ice blockage and a closed faucet. That pressure buildup can split the pipe at its weakest point, sometimes far from the frozen section. This is why the damage often isn’t discovered until the ice thaws and water starts flowing through the new gap.
How Freezing Water Breaks Rock
The same force that destroys plumbing also reshapes landscapes. When water seeps into cracks in rock and then freezes, the 9% expansion wedges the crack wider. Over repeated freeze-thaw cycles, this process, called frost wedging, can split boulders apart and gradually turn solid rock into gravel and soil.
Research on frost weathering has found that the process involves more than simple expansion. As ice forms inside rock, it also pulls unfrozen water toward the freezing front through a suction effect, adding even more pressure. In experiments on 47 different rock samples subjected to repeated freeze-thaw cycles, researchers measured progressive structural breakdown over time. Softer volcanic rocks were more vulnerable to the water migration effect, while harder rocks like andesite broke down primarily from the raw expansion force. This process is one of the main drivers of erosion in mountainous and cold-climate regions, and it’s the same mechanism behind pothole formation on roads.
Pressure Changes the Rules
Under high pressure, water’s freezing point drops below 0°C. This makes intuitive sense: since ice takes up more space than liquid water, squeezing the water harder makes it “prefer” to stay in its more compact liquid form. You need to cool it further to overcome that resistance and force it into the expanded crystal structure.
Dissolved gases also play a role. At higher pressures, more air dissolves into water, and those dissolved molecules further lower the freezing point. This is why deep ocean water can remain liquid at temperatures slightly below 0°C.
Most Substances Don’t Do This
Water’s expansion upon freezing is genuinely rare. The vast majority of materials are denser as solids than as liquids. A handful of other substances share this property, including bismuth, gallium, germanium, and silicon, but among common, everyday materials, water stands alone. This anomaly is entirely a product of hydrogen bonding geometry. Without that specific tetrahedral arrangement forcing gaps into the crystal, ice would sink, lakes would freeze from the bottom up, and cold-climate ecosystems would look radically different.