Diamond cannot be used

Analyze the question: "Diamond cannot be used..." asks which application is NOT a use of diamond. Three options are real uses and one is not.

Concept: Diamond is an allotrope of carbon in which each carbon atom is covalently bonded to four others in a rigid, giant tetrahedral lattice. This gives it:

• Extreme hardness (the hardest natural substance)
• A very high melting point
• Brittleness (it can cleave or shatter under a sharp blow)
• No free electrons, so it is a poor electrical conductor

Checking each option:

• A. In making bicycle chains: Bicycle chains need toughness, flexibility and strength under tension, which steel provides. Diamond is hard but brittle, and it cannot be shaped into flexible links. It is also far too expensive and unsuitable for this job. So diamond cannot be used here.
• B. As abrasives: Because of its hardness, diamond powder is used for grinding and polishing. This is a valid use.
• C. In cutting glass and metals: Diamond-tipped cutters and drill bits are used for glass, rock and metals. This is a valid use.
• D. As dies for drawing wires: Diamond dies are used to draw very fine wires because they resist wear. This is a valid use.

Answer: Options B, C and D all rely on diamond's hardness, so the exception is A.

Exam tip: For "cannot be used" questions on diamond, ask whether the use depends on hardness. If it does, diamond works. If it needs flexibility, conductivity or toughness, diamond does not (graphite conducts electricity and is used as a lubricant and in pencils, but diamond is not).