The density of ice and water as a function of temperature
The solid form of most substances is denser than the liquid phase; therefore, a block of a given solid will generally sink in its corresponding liquid. However, a block of ice floats in liquid water because ice is less dense than liquid water. The inset shows the curve in more detail in the range of 0-10 degrees Celsius. Liquid water is most dense at 4 degrees Celsius.
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