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

Question

The substance not likely to contain \(\mathrm{CaCO}_{3}\) is \([\mathbf{2 0 0 3}]\) (a) sea shells (b) calcined gypsum (c) dolomite (d) a marble statue

Step-by-Step Solution

Answer

Calcined gypsum is the substance not likely to contain CaCO3.

Step 1: Understand the options

Read and comprehend each option provided: (a) sea shells, (b) calcined gypsum, (c) dolomite, (d) a marble statue.

Step 2: Identify common substances containing CaCO3

Recall that sea shells are typically composed of calcium carbonate (CaCO3). Marble statues are also primarily made of calcium carbonate. Dolomite contains calcium magnesium carbonate which includes CaCO3 as part of its structure.

Step 3: Analyze calcined gypsum

Calcined gypsum is a product formed by heating gypsum, which is primarily composed of calcium sulfate dihydrate (CaSO4·2H2O). During calcination, gypsum loses water molecules to become calcium sulfate (CaSO4), not CaCO3.

Step 4: Determine the substance least likely to contain CaCO3

Based on our analysis, calcined gypsum does not contain calcium carbonate as its chemical composition is primarily calcium sulfate.

Key Concepts

Chemical Composition of Calcined Gypsum

Calcined gypsum is a significant industrial material used in construction and other applications. It originates from the mineral gypsum, which has the chemical formula \( \text{CaSO}_4 \cdot 2\text{H}_2\text{O} \). In its natural state, gypsum contains calcium sulfate dihydrate. However, it undergoes a process called calcination, where it is subjected to a controlled heat treatment.
During calcination, gypsum is heated to temperatures that drive off the water molecules, converting it to calcium sulfate hemihydrate, also known as plaster of Paris, with the formula \( \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} \).
- Calcined gypsum contains no \( \text{CaCO}_3 \) (calcium carbonate) as it primarily includes \( \text{CaSO}_4 \). - The absence of \( \text{CaCO}_3 \) distinguishes it from other substances like sea shells and marble, which are rich in calcium carbonate.
Its lack of \( \text{CaCO}_3 \) means it does not possess the same effervescent reaction with acids as substances containing calcium carbonate do.

Properties of Dolomite

Dolomite is a carbonate mineral that blends both calcium and magnesium in its crystalline structure. It is chemically represented by \( \text{CaMg(CO}_3)_2 \), reflecting its unique composition. This dual composition gives dolomite distinctive qualities compared to pure calcium carbonate minerals like limestone.
- Dolomite’s mix of calcium carbonate (\( \text{CaCO}_3 \)) and magnesium carbonate (\( \text{MgCO}_3 \)) provides it a more complex chemistry.- It is slightly less soluble in water than pure calcium carbonate and reacts slower when exposed to acids.
Dolomite is widely used as an ornamental stone, a source of magnesium oxide, and in various industrial applications like cement and brick production. Its double carbonate formation makes it especially suitable for applications where both calcium and magnesium are required.
The presence of both calcium and magnesium makes dolomite distinguishable from other carbonate structures, bringing forth its versatility and diverse usage in industries.

Calcium Carbonate in Sea Shells

Sea shells are fascinating natural forms composed primarily of calcium carbonate (\( \text{CaCO}_3 \)). They serve as protective structures for various marine organisms such as mollusks. The calcium carbonate in shells occurs in one of two crystalline forms: calcite or aragonite, both of which influence shell characteristics.
- The consistent calcium carbonate composition makes shells hard and durable, offering protection against predators and environmental changes. - Its chemical structure means that when exposed to acids, shells effervesce due to the release of carbon dioxide.
Sea shells not only contribute to marine biodiversity but also participate in ecosystems by helping to regulate levels of calcium and carbon in ocean waters.
In human industries, shells are utilized in artwork, jewelry, and even as natural fertilizers. The wide variety in shell shape and form reflects the diversity of species inhabiting them and showcases the remarkable adaptability of living organisms to their environments.