What do thermoplastics do when exposed to heat?

Thermoplastics, when exposed to heat, respond by softening and can reach a melting point where they transition into a pliable, malleable state. This property is crucial for various applications, including manufacturing processes such as molding and extrusion, where thermoplastic materials are shaped and formed into desired products.

The ability of thermoplastics to soften upon heating allows them to be repeatedly softened and reshaped without undergoing any permanent chemical change. This means that they can be recycled and remolded multiple times, making them versatile materials in design and engineering.

The other options do not accurately describe the behavior of thermoplastics under heat. They do not harden permanently, nor do they specifically become more elastic or combust spontaneously solely due to heat exposure. Instead, the melting and softening characteristic is what distinguishes thermoplastics from other types of polymers, highlighting their unique functional properties in practical applications.