Let's understand the difference between two main types of plastics: thermoplastics and thermosetting plastics. This will help us identify the correct example of a thermoplastic among the given options.

Understanding Thermoplastics and Thermosetting Plastics

Polymers can be broadly classified based on how they behave upon heating.

• Thermoplastics: These polymers soften on heating and can be moulded into different shapes. Upon cooling, they become hard again. This process of softening on heating and hardening on cooling can be repeated multiple times without significant change in their properties. Their polymer chains are typically held together by relatively weak intermolecular forces. Examples include polythene, PVC, nylon, etc.
• Thermosetting Plastics: These polymers undergo permanent chemical changes upon heating and moulding. Once heated and set into a shape, they become rigid and cannot be softened or remoulded by heating again. The polymer chains in thermosetting plastics are cross-linked, forming a rigid three-dimensional network structure. Examples include Bakelite, Melamine, Urea-formaldehyde resin, etc.

Analyzing the Options to Identify the Thermoplastic

Now let's look at each option:

• Bakelite: Bakelite is a type of thermosetting plastic. It is formed by the condensation reaction of phenol and formaldehyde. It is known for its hardness, rigidity, and electrical insulation properties, and it does not soften upon heating after being set.
• Melamine: Melamine formaldehyde resin is also a thermosetting plastic. It is used in making unbreakable dinnerware and decorative laminates. Like Bakelite, it forms a rigid cross-linked structure upon heating during its formation and cannot be remoulded.
• Polythene: Polythene (also known as polyethylene) is a common type of plastic used in packaging, films, bottles, etc. It softens when heated and can be easily moulded. Upon cooling, it hardens. This characteristic behaviour confirms that polythene is a thermoplastic polymer.
• Urea-formaldehyde resin: This is another example of a thermosetting plastic. It is used in adhesives, laminates, and electrical fittings. It forms a rigid, cross-linked structure that does not soften upon reheating.

Based on their behaviour upon heating, we can clearly see that Polythene fits the definition of a thermoplastic, while the other options are thermosetting plastics. Therefore, Polythene is the correct example of a thermoplastic from the given list.