Which of the following are the forms of calcium carbonate? 1. Marble 2. Chalk 3. Limestone Select the answer using the code given below :

The correct answer is D, 1, 2 and 3. Marble, chalk and limestone are all made of the same compound, calcium carbonate, with the formula CaCO3. They differ only in how they were formed. Limestone is a sedimentary rock built from shells and skeletons of sea creatures and from calcite that settled out of sea water. Chalk is a soft, very fine-grained and porous kind of limestone made mostly of the tiny shells of marine plankton. Marble is a metamorphic rock: limestone that was heated and pressed deep inside the earth until its grains recrystallised into a hard, shiny stone. All three fizz with dilute hydrochloric acid and give off carbon dioxide, and all three give quicklime when heated strongly. Option A is wrong because it leaves out limestone. Option B is wrong because it leaves out marble. Option C is wrong because it leaves out chalk. Exam tip: limestone is sedimentary, marble is metamorphic, and both, with chalk, are CaCO3.