There are interactions between molecules in both liquid and solid forms. The energy that would be increasing the temperature of the solid past the melting point instead goes into changing the bonds from “solid bonds” to “liquid bonds”. Likewise, when liquid is boiling, the “liquid bonds” are being broken, releasing the molecules as gas, instead of the liquid being heated past the boiling point.
I take it you mean at the point at where it melts and not the entire process.
It’s because at a certain point any extra heat you add will cause molecules to change from being part of the solid to leaving the solid. The extra energy causes them to go bye bye rather than raising the temperature because they have enough energy to now leave.
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