Evaluating properties of graphite and SiO₂

Steps:

• Identify SiO₂ as a giant covalent structure with strong Si-O bonds, common in minerals like quartz.
• Recognize graphite as layered carbon sheets with covalent C-C bonds within layers and weak van der Waals forces between.
• Note SiO₂ occurs as silica impurity in iron ores such as hematite.
• Assess each statement against these structures and real-world uses.

Why A is correct:

• SiO₂ (silica) is a principal impurity in iron ore, forming slag with calcium oxide during extraction, as per metallurgical processes.

Why the others are wrong:

• B: High melting points result from strong covalent network bonds in both, not ionic bonds (SiO₂ and graphite are covalent).
• C: Si-O bond angles in SiO₂ are tetrahedral (109.5°), while graphite has trigonal planar C-C angles (120°).
• D: Lubrication occurs via sliding between layers due to weak van der Waals forces, not breaking covalent bonds.

Final answer: A