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Water Expands When It Freezes, Unlike Most Substances
Water is one of the few substances that expands when it freezes, increasing in volume by about 9% and becoming less dense as ice.
About this fact
Most substances become denser when they freeze, but water is unusual because ice is actually less dense than liquid water. This happens because water molecules form a hexagonal crystal structure when frozen, creating more space between molecules than in liquid form. This expansion by about 9% is why water pipes burst in winter and why ice floats on water. This unique property is crucial for life on Earth - if ice were denser than water, lakes and oceans would freeze from the bottom up, likely making complex life impossible.