UPSC NDA I and II 2020 General Ability Solved Paper

Concept:Both chalk and marble are naturally occurring forms of calcium carbonate ($\mathrm{CaCO_3}$).Explanation:Calcium carbonate is a common substance found in rocks. Chalk is a soft, white, porous form of calcium carbonate. It forms from the accumulation of tiny marine organism shells over millions of years. Pressure and heat from Earth's movements transform chalk into harder limestone. Further heat and pressure over longer periods turn limestone into marble, which is a much denser and harder crystalline form. Thus, chalk and marble are simply different physical forms (allotropes or polymorphs) of the same chemical compound, calcium carbonate.Answer:Calcium carbonate