Phase changes of water

You need to add heat to melt ice and evaporate water. In the other direction, as water condenses from vapour to liquid and as it freezes from liquid to ice, heat is released.

So if water droplets condense from vapour in the air, heat is released, warming the air. If water droplets freeze into ice, heat is also released, warming the air. On the other hand, if ice (snowflakes or hail, for example) melts, heat is absorbed from the air, cooling it. If water droplets in the air evaporate, again heat is absorbed, cooling the air – this process is called evaporational cooling.

There is also a shortcut: water can transition directly from solid to vapour – this process is called sublimation. Energy-wise it is the same as other processes: it takes energy to evaporate ice, and energy is released when vapour goes directly into ice.

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