Chalk and marble are different forms of
- A.Calcium hydrogen carbonate
- B.Calcium carbonate
- C.Calcium acetate
- D.Sodium carbonate
Correct answer
Explanation
The correct answer is B, Calcium carbonate. Chalk, limestone and marble are all forms of calcium carbonate, CaCO₃.
Chalk is a soft, porous rock built from the tiny shells of marine plankton. Limestone is a harder sedimentary rock of the same compound, and marble forms when limestone is changed by heat and pressure deep in the earth, so it is a metamorphic rock. All three fizz with dilute hydrochloric acid, giving off carbon dioxide. Heating calcium carbonate gives quicklime (CaO), and this is why marble monuments such as the Taj Mahal are harmed by acid rain.
A is wrong because calcium hydrogen carbonate, Ca(HCO₃)₂, exists only in solution and causes temporary hardness of water.
C is wrong because calcium acetate is a salt of acetic acid, used as a food additive and in medicine.
D is wrong because sodium carbonate is washing soda (decahydrate) or soda ash, a sodium compound.
Exam tip: CaCO₃ – chalk, limestone, marble; CaO – quicklime; Ca(OH)₂ – slaked lime; CaSO₄·½H₂O – plaster of Paris.
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