Final answer:
Amorphous materials exhibit poorly defined fusion points due to variable intermolecular attractions, leading to a gradual softening instead of a distinct melting point.
Step-by-step explanation:
Substances that show poorly defined fusion points are typically amorphous materials. Unlike crystalline solids, which have precise melting temperatures due to the uniform strength of attractive forces holding their particles together, amorphous materials display a range of melting temperatures.
This happens because the structural nonequivalence of the molecules in an amorphous solid leads to variations in the strength of intermolecular forces. As the temperature increases, the weakest forces break first, leading to a gradual softening rather than a distinct melting point. These properties arise due to the variable intermolecular attractions within the substance.