Derretirse vs. descongelarse

In Spanish, 'derretirse' and 'descongelarse' are commonly used to express the transformation of something from a solid to a liquid state. However, they are not used interchangeably due to their nuanced differences in meaning.

Derretirse

'Derretirse' generally means 'to melt'. It is used in reference to a change in state without the need for pre-freezing. It is often used in the context of high heat causing an item to become liquid such as ice turning into water or chocolate turning gooey.
El hielo se derrite rápidamente en el sol.
(The ice melts quickly in the sun.)
El queso se derritió en el pan caliente.
(The cheese melted on the hot bread.)
Mi corazón se derrite cuando veo a un cachorro.
(My heart melts when I see a puppy.)

Descongelarse

'Descongelarse' translates to 'to thaw' or 'defrost'. It specifically implies that something was previously frozen and is now returning to its original, non-frozen state. This term is usually used in context of food or other items that have been preserved by freezing.
Es necesario descongelar el pollo antes de cocinarlo.
(You need to thaw the chicken before cooking it.)
La carne debe ser descongelada correctamente para evitar la proliferación de bacterias.
(Meat must be properly defrosted to prevent the growth of bacteria.)
Déjame descongelar las fresas para la ensalada de frutas.
(Let me thaw the strawberries for the fruit salad.)

Summary

While both 'derretirse' and 'descongelarse' refer to changing from a solid to a liquid state, 'derretirse' is a general term for melting caused by heat while 'descongelarse' specifically refers to thawing out something previously frozen.