Final answer:
An amorphous solid becomes softer when heated due to its lack of a precise melting temperature and the varying strengths of molecular attraction within it.
Step-by-step explanation:
The type of solid that becomes softer or changes to another substance when heated is an amorphous solid. Amorphous solids do not have a precise melting temperature like crystalline solids do. Instead, they have a range over which they soften because their molecular makeup is less ordered. This lack of order means different parts of the material are held together with different strengths of attraction. When heated, these weaker forces break first, causing the material to soften gradually as more heat is applied and stronger forces are also broken.