Thermoplastic materials become ________ when heated.

Thermoplastic materials are characterized by their ability to soften when heated and become pliable, allowing them to be molded or shaped. This property is fundamental to thermoplastics because, upon cooling, they solidify again into a rigid structure. This reversible process of softening and hardening is valuable in manufacturing and prototyping because it allows for reshaping and reuse of the material.

The relevant properties of thermoplastics highlight that they do not become hard or rigid when heated. Instead, they lose structure and become soft, which is essential for processing techniques such as injection molding or 3D printing. Additionally, the weight of the material does not change significantly with heating; thus, the option concerning heaviness is not appropriate. Understanding this behavior of thermoplastics is crucial for applications in various fields where versatility and reusability are desired.