Anomalous Expansion Of Water

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During winter a lake freezes only at its surface while fish survive in the water below; this is best explained by which unusual thermal property of water near 4°C?

RankGuru · Accepted Answer

As described in NCERT Class 11, Chapter 11 (Thermal Properties of Matter), water shows anomalous expansion: as it cools from higher temperatures its volume decreases until about 4°C, where its density is maximum, and on further cooling toward 0°C it expands again. This unusual behaviour means the densest water sinks to the bottom while colder, lighter water rises to the surface and eventually freezes there. The layer of ice on top insulates the water beneath, keeping it liquid so aquatic life can survive. The option claiming minimum density at 4°C reverses the true behaviour. The option of uniform expansion at all temperatures ignores the anomaly entirely. The option of zero expansion below 4°C is false, since water actually expands as it approaches freezing. A consistency check confirms the everyday observation that ice floats and lakes freeze top-down, both direct consequences of water's density maximum near 4°C.

Anomalous Expansion Of Water

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