Calcium Oxide, Calcium Carbonate, Plaster of Paris — Core Principles

Core Principles

Calcium Oxide (CaO), known as quicklime, is a highly reactive basic oxide produced by heating calcium carbonate. It readily reacts with water in an exothermic process called slaking to form calcium hydroxide (slaked lime), Ca(OH)$_2$.

Quicklime is crucial in cement manufacturing, metallurgy as a flux, and as a drying agent due to its strong affinity for water. Calcium Carbonate (CaCO$_3$) is a widely occurring mineral found as limestone, marble, and chalk.

It decomposes upon heating to yield quicklime and carbon dioxide. It reacts with acids to release CO$_2$. Its applications span construction, antacids, and as a raw material for various industries. Plaster of Paris (CaSO$_4 \cdot \frac{1}{2}$H$_2$O), or calcium sulfate hemihydrate, is derived from gypsum (CaSO$_4 \cdot 2$H$_2$O) by heating it to $100^\circ\text{C}$.

Its defining characteristic is its ability to set into a hard mass upon mixing with water, forming gypsum again. This property makes it invaluable for medical casts, dental impressions, and decorative molds.

Overheating gypsum leads to 'dead burnt plaster' (anhydrous CaSO$_4$), which loses the setting property.

Often confused with

Side-by-side differences the NEET paper likes to test.

Calcium Oxide, Calcium Carbonate, Plaster of Paris vs Gypsum and Plaster of Paris

Aspect	Calcium Oxide, Calcium Carbonate, Plaster of Paris	Gypsum and Plaster of Paris
Chemical Formula	Gypsum: CaSO$_4 \cdot 2$H$_2$O	Plaster of Paris: CaSO$_4 \cdot \frac{1}{2}$H$_2$O
Water of Crystallization	Contains two molecules of water.	Contains half a molecule of water.
Preparation	Naturally occurring mineral.	Prepared by heating gypsum to $100^\circ\text{C}$.
Setting Property	Does not set with water.	Sets into a hard mass when mixed with water.
State	Solid mineral, relatively soft.	Fine white powder.

Gypsum is the naturally occurring dihydrate of calcium sulfate, containing two molecules of water of crystallization. It is the raw material for Plaster of Paris. Plaster of Paris, or calcium sulfate hemihydrate, is produced by partially dehydrating gypsum at $100^\circ\text{C}$, retaining only half a molecule of water. The key difference lies in their water content and, consequently, their ability to set with water. PoP readily rehydrates and sets, while gypsum does not possess this property.

Why it is tested: NEET relevance: Understanding the precise chemical formulas, the conditions for conversion (especially the temperature for PoP formation), and the distinct properties like 'setting' and 'dead burnt plaster' are frequently tested. Questions often involve identifying the correct formula or the product formed under specific heating conditions.