Which of the following is a form of calcium carbonate? I. limestone II. marble III. chalk Iv. egg-shells

Analysis

The question asks us to identify which of the listed substances are forms of calcium carbonate ($CaCO_3$). We need to evaluate each item to see if it is chemically composed primarily of calcium carbonate.

Concept Explanation

Calcium carbonate is a naturally occurring chemical compound. It is the main component of many geological formations and biological structures.

• Limestone: A sedimentary rock composed mainly of skeletal fragments of marine organisms such as coral and mollusks. It is essentially calcium carbonate.
• Marble: A metamorphic rock formed when limestone is subjected to high heat and pressure. It is a recrystallized form of calcium carbonate.
• Chalk: A soft, white, porous sedimentary carbonate rock, a form of limestone composed of the mineral calcite ($CaCO_3$).
• Egg-shells: The hard outer covering of the eggs of birds and some reptiles. It is composed primarily of calcium carbonate crystals stabilized by a protein matrix.

Step-by-Step Solution

1. Evaluate I (Limestone): Limestone is widely known as a sedimentary rock consisting of $CaCO_3$. (Correct)
2. Evaluate II (Marble): Marble is metamorphosed limestone, meaning it retains the chemical composition of $CaCO_3$. (Correct)
3. Evaluate III (Chalk): Chalk is a specific, soft variety of limestone, which is $CaCO_3$. (Correct)
4. Evaluate IV (Egg-shells): Biological studies confirm that the primary mineral component of an eggshell is calcium carbonate. (Correct)

Since all four items (I, II, III, and IV) are forms or sources of calcium carbonate, they all satisfy the condition of the question.

Conclusion

Comparing our findings with the provided options, we see that all four items are included in option D.

Exam Tip: When you see a question listing multiple items like this, remember that calcium carbonate is very common in nature. If you recognize that most of the items are definitely $CaCO_3$, look for an option that includes all of them rather than trying to eliminate one unless you are certain it is chemically different.