Which of the following is an example of a thermosetting plastic, that is, a polymer which once moulded and hardened cannot be softened by reheating?

Introduction / Context:
Plastics are broadly divided into thermoplastics and thermosetting plastics. Thermoplastics soften on heating and can be remoulded, while thermosetting plastics undergo a permanent hardening process and cannot be softened again by heat. This question asks for an example of a thermosetting plastic. Knowing the difference between these two classes is important in materials science and everyday applications.

Given Data / Assumptions:

• We are comparing common plastics: Bakelite, PVC, polyethylene and others.
• Thermosetting plastics harden irreversibly when first moulded.
• Thermoplastics can be repeatedly softened and reshaped by heating.
• The correct answer must be a thermosetting plastic.

Concept / Approach:
Bakelite is a classic example of a thermosetting plastic. It is a phenol formaldehyde resin that forms an extensively cross linked network when cured. Once this network is formed, Bakelite does not soften on reheating and keeps its shape even at relatively high temperatures. In contrast, PVC (polyvinyl chloride), polyethylene, and polystyrene are thermoplastics that soften when heated and can be reshaped multiple times. Therefore, among the options given, Bakelite is the thermosetting plastic.

Step-by-Step Solution:

Verification / Alternative check:
Materials science references list Bakelite as one of the earliest synthetic plastics and emphasise that it is thermosetting. Historical descriptions explain how Bakelite revolutionised electrical insulation because it did not soften when devices became warm. In contrast, PVC is commonly used for pipes and cable insulation that can be moulded by heat, polyethylene for bags and bottles, and polystyrene for foam packaging, all of which are reprocessable by heating. These examples confirm the classification of Bakelite as thermosetting and of the others as thermoplastics.

Why Other Options Are Wrong:

• PVC: Polyvinyl chloride is a thermoplastic and softens on heating, allowing it to be extruded into pipes and other shapes.
• Polyethylene: A typical thermoplastic used for carry bags and bottles that can be melted and reshaped.
• No option is correct.: Incorrect because Bakelite is indeed a thermosetting plastic.
• Polystyrene: Another thermoplastic used in foam products and disposable cups that softens when heated.

Common Pitfalls:
Students sometimes assume that all plastics behave similarly when heated, not realising that thermosetting plastics are fundamentally different from thermoplastics. Another error is to confuse Bakelite with more modern plastics such as PVC. One way to remember the difference is to associate Bakelite with old style electrical switches and handles that remain hard and brittle, indicating a thermosetting nature, while PVC and polyethylene are flexibile and can be softened or welded with heat.

Final Answer:
An example of a thermosetting plastic is Bakelite.