Why is water strange in thermal expansion?

Answer 1

Unlike most substances, when water freezes, it forms a structure that is less dense than the liquid it is in equilibrium with (which is why ice floats on liquid water). It is also unusual in that the maximum density of the liquid reaches a minimum at a temperature ABOVE the freezing point - roughly 1.0000 g/cm3 at 4 °C - before it begins to increase as it approaches the freezing point. Water thus has a negative coefficient of thermal expansion between 0 °C and 4 °C and a zero coefficient right at 4 °C. Other substances have uniformly positive coefficients of thermal expansion.

Answer 2

Water is considered strange in thermal expansion because unlike most substances, it expands when it freezes. This is due to the unique crystalline structure of ice, which causes the molecules to form a lattice with a more open structure than liquid water, resulting in increased volume.