Graphite and Diamond are similar in that they

Analyze the question: We're asked what graphite and diamond have in common. Both are allotropes of carbon, meaning they are different physical forms of the same element.

Concept: Allotropes have the same chemical properties (because they contain the same atoms) but different physical properties (because their atoms are arranged differently).

• Diamond: Each carbon atom is bonded tetrahedrally to four others in a rigid 3D giant structure. It is very hard, has a high density (about 3.5 g/cm³), and does not conduct electricity.
• Graphite: Carbon atoms are arranged in flat hexagonal layers held together by weak van der Waals forces. It is soft and slippery, has a lower density (about 2.3 g/cm³), and conducts electricity through its delocalised electrons.

Check each option:

• A. Octahedral shape: Diamond's crystals can be octahedral, but graphite forms hexagonal layers, so this is not a shared feature. ✗
• B. Same density: Diamond is denser than graphite. ✗
• C. Form carbon(IV) oxide on combustion: Both are pure carbon, so when burned in excess oxygen, both give CO₂. ✓ C + O₂ → CO₂
• D. Conduct electricity: Only graphite does. Diamond has no free electrons. ✗

Answer: C

Exam tip: For "allotropes are similar in..." questions, think chemical properties are the same, physical properties differ. Anything involving a chemical reaction (such as combustion products) is usually the shared feature, while hardness, density, and conductivity usually differ.