Problem 71 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is dead burnt plaster? (a) anhydrous \(\mathrm{CaSO}_{4}\) (b) \(\mathrm{CaSO}_{3}\) (c) \(\left(\mathrm{CaSO}_{4}\right)_{2} \cdot \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{CaSO}_{4} \cdot 2 \mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

(a) anhydrous \(\mathrm{CaSO}_{4}\)

Step 1: Understand the Concept

Dead burnt plaster is a form of plaster that has been heated to remove the majority, if not all, of its water content, leaving mainly the anhydrous compound. To solve the problem, identify which chemical formula represents this.

Step 2: Analyze Each Option

Examine each compound: - Option (a) anhydrous \(\mathrm{CaSO}_{4}\) is the form of plaster where all the water has been removed. - Option (b) \(\mathrm{CaSO}_{3}\) is calcium sulfite, not related to plaster. - Option (c) \(\left(\mathrm{CaSO}_{4}\right)_{2} \cdot \mathrm{H}_{2} \mathrm{O}\) denotes \'half-hydrate\' or \'hemihydrate\', not completely decomposed. - Option (d) \(\mathrm{CaSO}_{4} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) is gypsum, fully hydrated and not dead burnt.

Step 3: Identify the Correct Option

Based on our analysis, anhydrous \(\mathrm{CaSO}_{4}\) is the only compound that is classified as dead burnt plaster because it lacks any water content after being strongly heated to drive out water.

Key Concepts

Anhydrous Calcium Sulfate

Anhydrous calcium sulfate is a chemical compound known for being extremely dry, lacking any water molecules in its structure. The term "anhydrous" itself signifies the absence of water content.
Calcium sulfate, a compound with the chemical formula \(\mathrm{CaSO}_{4}\), becomes anhydrous when it is heated to high temperatures. This heating process removes any water attached to it, leaving a fine, white, dry powder known as "dead burnt plaster."

• This compound is commonly used in building materials due to its optimal properties when mixed with water, such as setting into a hard substance.
• Its anhydrous nature makes it ideal for scenarios where moisture presence needs to be minimalized.

Applications also extend to industries requiring dehumidification and as a drying agent. Its high purity and ability to adsorb water make it a staple where dry materials are crucial.

Gypsum

Gypsum is a naturally occurring mineral, primarily composed of calcium sulfate dihydrate, indicated by the formula \(\mathrm{CaSO}_{4} \cdot 2 \mathrm{H}_{2} \mathrm{O}\).
This means each molecule of calcium sulfate in gypsum is bonded with two molecules of water. It's a soft mineral, often used in the construction industry for producing plaster and wallboard.

• Being predominantly made of hydrated calcium sulfate, gypsum plays a crucial role in drywall manufacturing due to its fire-resistant properties.
• When heated above 100 degrees Celsius, the water content starts to evaporate, eventually forming anhydrous calcium sulfate.

Gypsum is also known for its application in agriculture as a soil conditioner, helping improve water infiltration and aeration of clay-heavy soils.

Chemical Formula Analysis

Chemical formula analysis involves studying the composition and structure of a compound by interpreting its chemical formula. In the context of this exercise, analyzing formulas helps identify the type of compound based on its water content.
A compound’s formula gives insights into its molecular components and ratios. For example:

• \(\mathrm{CaSO}_{4}\) indicates calcium, sulfur, and oxygen in a 1:1 ratio without water, making it anhydrous.
• \(\mathrm{CaSO}_{4} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) reveals calcium sulfate with water molecules, known as gypsum.

By breaking down each formula, one can deduce that option (a) from the exercise is "dead burnt plaster" because it lacks water, unlike the other options, which have varying hydration levels. This analytical skill is critical in chemistry for identifying material properties and potential applications.