Polythene, also called polyethylene, is one of the most common plastics. It is formed by the polymerisation of which monomer?

Introduction / Context:
This question focuses on a very common plastic, polythene or polyethylene, and asks you to recall its basic building block monomer. Understanding which monomer gives rise to which polymer is a core idea in polymer chemistry. Polythene is widely used in carry bags, bottles and packaging films, so knowing its monomer helps in understanding its production, structure and properties.

Given Data / Assumptions:

• The polymer of interest is polythene or polyethylene.
• It is produced by polymerisation of a single type of monomer.
• The options include ethylene, ethyl alcohol, vinyl chloride and ethanol.
• All options are common organic compounds but only one is the correct monomer.

Concept / Approach:
Polythene is an addition polymer formed by the polymerisation of ethylene, which is also called ethene and has the formula CH2=CH2. In addition polymerisation, the carbon carbon double bond in the monomer opens up and many monomer units join to form a long chain. Ethyl alcohol and ethanol are two names for the same compound, CH3CH2OH, which does not undergo simple addition polymerisation to give polythene. Vinyl chloride (CH2=CHCl) is the monomer for polyvinyl chloride, known as PVC, not for polythene. Recognising that ethylene is the correct monomer is the key to answering the question.

Step-by-Step Solution:
Step 1: Recall that polythene is also called polyethylene and is a polymer of ethene or ethylene. Step 2: Ethylene has a carbon carbon double bond that can open during polymerisation. Step 3: Under high pressure and appropriate conditions, many ethylene molecules join to form a long chain polymer with repeating –CH2–CH2– units. Step 4: Examine the options. Ethyl alcohol and ethanol refer to the same alcohol molecule, which is not used to make polythene. Step 5: Vinyl chloride is the monomer used to produce polyvinyl chloride, a different plastic. Step 6: Therefore, only ethylene matches the known monomer for polythene.

Verification / Alternative check:
Polymer chapters in textbooks often list common monomer polymer pairs: ethene to polyethene, propene to polypropylene, vinyl chloride to polyvinyl chloride, tetrafluoroethene to polytetrafluoroethene and so on. In all these cases, the monomer is a small unsaturated molecule with a double bond. Ethylene fits this pattern perfectly and is always linked to polythene in such tables. You can also recall that polyethylene is often represented structurally as (CH2–CH2)n, clearly showing that the monomer unit corresponds to CH2=CH2, which is ethylene.

Why Other Options Are Wrong:
Ethyl alcohol and ethanol are the same compound, an alcohol with an –OH group, and are not used for manufacturing polythene. They may undergo other types of reactions such as oxidation, dehydration or esterification, but not addition polymerisation to form polyethylene. Vinyl chloride, with a chlorine atom attached, is used to make polyvinyl chloride, which has different properties like rigidity and chemical resistance, unlike polythene. Thus these options do not match the well established link between ethylene and polythene.

Common Pitfalls:
A frequent confusion arises because students may mix up different common plastics and their monomers. For example, it is easy to confuse vinyl chloride and ethylene because both have a carbon carbon double bond and are gaseous at room temperature. Additionally, the presence of both ethyl alcohol and ethanol as options may seem to indicate one of them is correct, but they are actually the same compound and serve as distractors. To avoid such mistakes, memorize a short list of the most important monomer polymer combinations and always focus on the presence of double bonds for addition polymers like polythene.

Final Answer:
Polythene is obtained by addition polymerisation of the monomer ethylene.