Problem 167 - A Complete Resource Book in Chemistry for JEE Main

Question

The substance not likely to contain \(\mathrm{CaCO}_{3}\) is (a) sea shells (b) calcined gypsum (c) dolomite (d) a marble statue

Step-by-Step Solution

Answer

Calcined gypsum (b) is the substance not likely to contain \( \mathrm{CaCO}_{3} \).

Step 1: Understand Calcium Carbonate

Calcium carbonate is a chemical compound with the formula \( \mathrm{CaCO}_{3} \). It is commonly found in rocks as minerals such as calcite and aragonite, and main component of eggshells, seashells, and animal shells.

Step 2: Analyze Options

Let's analyze each of the given options: (a) Sea shells are primarily composed of calcium carbonate. (b) Calcined gypsum involves heating of gypsum, which is primarily composed of calcium sulfate (\( \mathrm{CaSO}_{4} \)), not calcium carbonate. (c) Dolomite is a mineral composed of calcium magnesium carbonate \(( \mathrm{CaMg(CO}_3)_2 )\), which means it contains \( \mathrm{CaCO}_{3} \). (d) Marble is a metamorphic rock largely composed of recrystallized carbonate minerals, typically calcite, hence it contains \( \mathrm{CaCO}_{3} \).

Step 3: Determine the Likely Exception

From Step 2, we see that calcined gypsum is the only option not predominantly composed of or lacking calcium carbonate. Instead, it is mainly known for its calcium sulfate content. Therefore, calcined gypsum is least likely to contain \( \mathrm{CaCO}_{3} \).

Key Concepts

Calcined Gypsum

Calcined gypsum is created by heating gypsum, often used in construction. Gypsum itself is a naturally occurring mineral made of calcium sulfate dihydrate

• Formula: \( \mathrm{CaSO}_4 \cdot 2\mathrm{H}_2\mathrm{O} \)

During the calcination process, gypsum is heated to around 150-180°C. This heat removes part of the water content, forming what is known as plaster of Paris. This material then predominantly consists of calcium sulfate hemihydrate

• Formula: \( \mathrm{CaSO}_4 \cdot \frac{1}{2}\mathrm{H}_2\mathrm{O} \)

It is important to note that calcined gypsum does not contain calcium carbonate. Hence, it does not relate chemically to limestone or other carbonate-based materials.

Sea Shells

Sea shells are natural containers produced by various marine organisms such as mollusks. These shells primarily consist of calcium carbonate.

• Types: Calcite and aragonite are the two main mineral forms
• Structure: The structure can vary but often has a smooth and hard surface

Sea shells form through a biological process in which the organism secretes substances to build its own protective structure. The calcium carbonate gives shells their rigidity and resilience. Over time, shells can accumulate and contribute to the formation of shell beds, which are important ecological structures in marine environments.

Dolomite

Dolomite is a mineral often confused with limestone. It is composed of calcium magnesium carbonate

• Formula: \( \mathrm{CaMg(CO}_3)_2 \)

This mineral is found in sedimentary rock formations and can form large, aesthetically pleasing crystals. Dolomite's double carbonate structure allows it to host both calcium and magnesium ions. Therefore, dolomite contains calcium carbonate, contributing to its importance in geological formations and industrial applications.
Additionally, the distinction between dolomite and limestone lies in the presence of magnesium, making dolomite distinct in both chemical makeup and formation processes.

Marble

Marble is a metamorphic rock renowned for its beauty and strength. It originates from limestone that has undergone metamorphism.

• Main Component: Recrystallized carbonate minerals, primarily calcite or dolomite
• Structure: Characterized by its crystalline texture and smooth surface

During the metamorphic process, the original calcite crystals in the limestone recrystallize to form the interlocking calcite crystals that give marble its characteristic beauty. This makes marble a popular choice in sculpture and architecture. Due to its calcium carbonate composition, marble can sometimes show distinguishing colored veining resulting from mineral impurities dispersed throughout."}]}]}]}}}]} In its pure form, marble is usually pale due to its calcite content, though impurities can add a range of hues.

Marble's high durability and aesthetic appeal make it sought after for use in flooring, facades, and detailed carvings, emphasizing its long-standing value as a material in art and construction.