Why Do We Call 32° Above Zero “Freezing”?
We know that freezing is the transformation of a liquid into solid under the influence of cold. Each liquid always solidifies at some fixed temperature, which is called its freezing point, and the solid melts again at the same temperature. Thus the freezing point and the melting point, or point of fusion, are the same, and the point is always the same for the same substance.
Consequently the freezing point of water, or the melting point of ice (32° F.), is taken for one of the fixed points in thermometry. The freezing point of mercury is 39° below zero, of sulphuric ether 46° below zero, of alcohol 203° below zero F.