How Everything Works
How Everything Works How Everything Works

Question 710

What happens to the temperature of water when ice is added? — OS, Havana, FL
Whenever a glass contains a mixture of both ice and water, the temperature of its contents will be 32° F (0° C). That's because liquid water isn't stable below this temperature and solid ice isn't stable above this temperature. The two can coexist stably only at 32° F.

When you first mix the water and ice, the water is likely to hotter than 32° F and the ice is likely to be colder than 32° F. Heat flows from the water to the ice—from hotter to colder—and soon one of them reaches 32° F. While heat may then continue to flow from the water to the ice, the one that's at 32° F won't change temperature any more. Instead, it will begin to turn into the other form. For example, if the ice reaches 32° F first, it will begin to melt as heat flows into it and turn into water at 32° F. Or if the water reaches 32° F first, it will begin to freeze as heat flows out of it and turn into ice at 32° F. Pretty soon, both the ice and the water will be at 32° F and there they will remain so long as both are still present in the glass.

From a molecular standpoint, ice is an orderly crystalline solid in which the water molecules are neatly arranged in rows, columns, and stacks. Liquid water is a disorganized soup of water molecules that are regularly changing neighbors and moving about, though always in contact with one another. As you add heat to cold ice, its molecules jiggle more and more vigorously against one another until they finally begin to move about as liquid water, a process we call "melting." As you remove heat from warm water, its molecules move about less and less rapidly until they finally begin to cling permanently to one another as solid ice, a process we call "freezing."


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