Does hot water freeze faster than cold?
No. But it could seem that way. Why? Because it takes a lot more cooling to make ice from water that’s at the freezing point (0 degrees Celsius) than to cool water from, say, 1 degree to 0 degrees. Notice how the graph stabilizes at 0 degrees Celsius. Water does not freeze until the heat of crystallization is removed. To change matter from the one state to another, you must add or remove heat energy. To cool one gram of water by one degree Celsius, you must remove one calorie of heat energy. But after the water reaches 0 degrees Celsius, freezing requires the removal of almost 80 calories — the heat of fusion. Similarly, you must add almost 80 calories to melt one gram of ice at 0 degrees Celsius. Why can it seem that hot water freezes faster than cold water? Because the bulk of the cooling serves to remove the heat of crystallization. So to freeze one gram of water at 80 degrees C, you must remove 160 calories, only twice as much as you need to freeze one gram at 0 degrees.
