UPSC NDA 2 2019 General Ability Solved Paper

Concept:Diamond and graphite are allotropes of carbon.Allotropes have different physical properties but identical chemical properties.Hence, the statement that both physical and chemical properties are different is incorrect.Explanation:Diamond and graphite are both pure carbon, so their chemical properties are the same (e.g., both burn to give $\mathrm{CO}_2$).Their physical properties differ due to different bonding and structures.• Diamond: each carbon is $sp^3$ hybridized, forming a tetrahedral network of strong covalent bonds. This makes it extremely hard and an electrical insulator.• Graphite: each carbon is $sp^2$ hybridized, forming hexagonal planar layers held by weak van der Waals forces. Layers slide easily, making it soft and slippery. The delocalized $\pi$ electrons allow it to conduct electricity.Thus, statement A (tetrahedral vs hexagonal planar), C (soft vs hard), and D (conductor vs insulator) are correct.Statement B claims both physical and chemical properties are different, which is false because chemical properties are the same.Answer:The incorrect statement is B: “Both physical and chemical properties of diamond and graphite are different.”