Water is one of the very few substances with anomalous thermal expansion. In most materials volume always increases with temperature, but between C and C water does the opposite: it contracts as increases, reaching maximum density at exactly C, and only beyond that threshold does it begin to expand like every other body.
Why ice floats and lakes freeze from the top
Since water is denser at C than at C, colder water (and ice, which is even less dense) stays at the surface. Lakes therefore freeze from the top down: beneath the ice crust, the water remains liquid at around C, safeguarding aquatic life throughout the winter. If water behaved “normally”, lakes would freeze from the bottom and many ecosystems would not survive.
This apparent exception is actually a consequence of the particular structure of water molecules and their hydrogen bonds, which near the freezing point tend to arrange themselves into a more “open” and therefore less dense lattice.
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Topics: Thermology Concepts: Thermal expansion
Related exercises: Worked exercise — The railway track in winter · Problem — Three bars at the same temperature · Problem — Aluminium with doubled lambda