Natural rubber can be made more strong, elastic, and bouncy through a process called vulcanization. Which substance is mainly added to natural rubber in this process to achieve these properties?

Introduction / Context:
Natural rubber in its raw form is soft, sticky, and not very stable over a wide range of temperatures. To make it useful for products like tyres, hoses, and shoe soles, it needs to be made stronger, more elastic, and more resistant to heat and wear. This improvement is achieved through a chemical process called vulcanization. The question asks you to identify which substance is mainly added to natural rubber during vulcanization to create cross links between polymer chains and improve its properties.

Given Data / Assumptions:
- Natural rubber is a polymer made from isoprene units, which can be chemically modified.
- Vulcanization is a standard industrial process used to strengthen rubber.
- The options include chlorine, sulphur, sponge, polythene, and glass fibres.
- Only one of these is widely known as the key vulcanizing agent for natural rubber.

Concept / Approach:
In vulcanization, sulphur atoms form cross links between long chains of natural rubber molecules. These sulphur cross links tie the chains together, limiting their mobility and making the material more elastic and resilient. As a result, vulcanized rubber returns to its original shape after being stretched and does not soften excessively in heat or become brittle in cold conditions. Chlorine and polythene have very different roles in chemistry and materials science, sponge is not a chemical but a material structure, and glass fibres are used for reinforcement in composites, not for vulcanizing rubber.

Step-by-Step Solution:
Step 1: Recall that vulcanization is associated historically with adding sulphur to natural rubber. Step 2: Evaluate option A, chlorine, which is used in chlorination processes but is not the main vulcanizing agent for rubber. Step 3: Evaluate option B, sulphur, which is well known as the primary chemical used to create cross links in natural rubber. Step 4: Evaluate option C, sponge, which refers to a porous material, not a chemical used in vulcanization. Step 5: Evaluate option D, polythene, a plastic polymer that is not used to vulcanize rubber. Step 6: Evaluate option E, glass fibres, which are used in fibre reinforced composites, not as cross linking agents. Step 7: Conclude that sulphur is the correct answer.

Verification / Alternative check:
Many textbooks and historical references state that Charles Goodyear discovered vulcanization by heating natural rubber with sulphur. The resulting material was less sticky and more durable, leading to the wide use of rubber in tyres and industrial products. Modern vulcanization often uses sulphur along with accelerators and activators, but sulphur remains the central element that forms cross links. This reinforces that sulphur is the correct substance to name in a general knowledge question about making rubber more strong and bouncy.

Why Other Options Are Wrong:
Chlorine, option A, can modify polymers but is not the standard cross linking agent used in the vulcanization of natural rubber.
Sponge, option C, describes a porous structure but is not a chemical agent and cannot form cross links in a rubber polymer.
Polythene, option D, is itself a plastic polymer and is not used to strengthen natural rubber through vulcanization.
Glass fibres, option E, are rigid reinforcing materials used in composites; they are not mixed into rubber to create molecular cross links in the vulcanization process.

Common Pitfalls:
A common error is to be distracted by the idea of reinforcement and imagine that adding strong materials like glass fibres directly to rubber automatically makes it better for all purposes. While fibre reinforcement works in some composites, vulcanization is a specific chemical process that depends on cross linking with sulphur. Another pitfall is confusing different chemical treatments such as chlorination with vulcanization, when in fact they serve different purposes.

Final Answer:
Sulphur is the substance mainly added to natural rubber during vulcanization to make it stronger, more elastic, and more bouncy.