Which statement best describes thermoplastics?

Thermoplastics are characterized as polymers with high molecular weights where the chains are associated through weak intermolecular forces rather than strong covalent bonds. This structural characteristic allows thermoplastics to soften when heated and can be reshaped upon cooling, which is a key feature of their behavior. The weak forces allow the polymer chains to slide past one another when thermal energy is applied, making them malleable and convenient for processes like molding. This is a distinctive property that differentiates them from thermosetting plastics, which form irreversible bonds and do not soften when heated.

The other options highlight characteristics that do not accurately reflect the behavior and structural composition of thermoplastics. Polymers with low molecular weights (as mentioned in the first option) would not typically exhibit the properties associated with thermoplastics. The mention of strong covalent bonds in the second option incorrectly suggests a rigidity that is not typical for thermoplastics. Lastly, polymers that do not alter their structure upon heating (the fourth option) describes thermosetting plastics, not thermoplastics. The correct understanding of thermoplastics centers on their ability to be reshaped through the application of heat due to the nature of the forces linking their molecular chains.