A 'phase' is a state of matter such as solid, liquid or gas.
To change phase, a material must absorb or release some heat. This heat is called 'latent heat'.
For instance, for liquid water at 100oC to change to steam at 100oC it must absorb some energy, even though it doesn't change temperature. It must then release that heat to change back to liquid.
This is the priniple on which phase change cooling is based: It can (theoretically) move heat at a temperature difference of zero.
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