| Freezing and melting coexist |
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Melting occurs when a substance changes from solid to liquid and freezing occurs when a substance changes from liquid to solid. From a molecular perspective, these two processes occur simultaneously. |
At 0°C, ice crystals gain and lose water molecules simultaneously |
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The temperature 0°C is both the melting temperature and the freezing temperature of water. At this temperature, water molecules in the liquid phase are moving slowly enough that they tend to cluster together to form ice crystals (freezing). At this same temperature, however, water molecules in ice are vibrating with great disorder, much more than they vibrate at colder temperatures. Many molecules thus escape from the crystalline structure to form liquid water (melting). Thus melting and freezing occur simultaneously.
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For water, 0°C is the special temperature at which the rate of ice formation equals the rate of liquid water formation. In other words, it is the temperature at which the opposite processes of melting and freezing counterbalance each other. This means that if a mixture of ice and liquid water is maintained at exactly 0°C, the two phases are able to coexist indefinitely.
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