The use of sulphur to vulcanize rubber is made to make it

Analyze the question: It asks why sulphur is used to vulcanize rubber, meaning what property the process improves.

Explain the concept: Natural rubber is soft, sticky, and weak. It becomes sticky when hot and brittle when cold. In vulcanization, rubber is heated with sulphur (about 1-3% by mass for ordinary rubber). The sulphur forms cross-links (disulphide or sulphur bridges) between the long polyisoprene chains. This changes the structure of the rubber.

Solution, step by step:

1. Raw rubber consists of long, coiled polymer chains that slide past each other easily. This makes it soft and weak, and it deforms permanently.
2. Heating with sulphur creates sulphur bridges between neighbouring chains.
3. The chains can no longer slip past each other easily. As a result, the rubber: • becomes harder and tougher • has greater tensile strength • resists wear, heat, and chemicals better • is no longer sticky • still returns to its shape after stretching
4. Checking the options: • A. Blacker: colour is not the purpose (carbon black is a separate additive). • B. Cheaper: cost is not the purpose. • C. More elastic: this is partly true, since vulcanized rubber recovers its shape better. But the main textbook answer is about strength and durability. • D. Tougher: this matches the main effect of cross-linking, which gives strength and durability. • E. Softer: wrong, because vulcanization makes rubber harder.

Option C is tempting because vulcanized rubber does have better elastic recovery. However, the standard reason given in Nigerian exam texts is that vulcanization makes rubber tougher, harder, and more durable.

Answer: D, tougher.

Tip: Remember "sulphur bridges = strong rubber." Vulcanization makes rubber tougher and more durable, and it does not make it softer or stickier.