Chemistry·Explained

Calcium Oxide, Calcium Carbonate, Plaster of Paris — Explained

Q: What happens if gypsum is heated above $100^\circ\text{C}$?

If gypsum (CaSO$_4 \cdot 2$H$_2$O) is heated above $100^\circ\text{C}$ (specifically around $200^\circ\text{C}$ or higher), it loses all its water of crystallization. This results in the formation of anhydrous calcium sulfate (CaSO$_4$), which is known as 'dead burnt plaster'. Unlike Plaster of Paris, dead burnt plaster loses its ability to set with water because its crystalline structure is irreversibly destroyed, preventing proper rehydration and interlocking crystal formation.

Q: How does Plaster of Paris set when mixed with water?

When Plaster of Paris (calcium sulfate hemihydrate, CaSO$_4 \cdot \frac{1}{2}$H$_2$O) is mixed with water, it rehydrates to form calcium sulfate dihydrate (gypsum, CaSO$_4 \cdot 2$H$_2$O). This rehydration process leads to the formation of numerous tiny, needle-like gypsum crystals. These crystals grow and interlock with each other, forming a rigid, solid, and strong crystalline matrix. This interlocking network is what gives the set plaster its hardness and structural integrity.

Q: What are the main industrial uses of calcium carbonate?

Calcium carbonate (CaCO$_3$) has numerous industrial applications. It is primarily used as a raw material for the production of quicklime (CaO) and cement. In construction, it's used directly as limestone and marble. It also serves as a filler in the paper, plastics, and paint industries, improving product properties. Furthermore, it's used as an antacid in pharmaceuticals, an abrasive in toothpaste, and as a dietary calcium supplement.

NEET UG

Updated 22 Mar 2026

Detailed Explanation

The chemistry of calcium compounds, particularly calcium oxide, calcium carbonate, and Plaster of Paris, forms a cornerstone of inorganic chemistry due to their natural abundance, diverse applications, and fundamental chemical principles. A thorough understanding of their preparation, properties, and reactions is essential for NEET aspirants.

I. Calcium Oxide (CaO) - Quicklime

Conceptual Foundation: Calcium oxide, commonly known as quicklime, is a highly basic oxide of calcium. Its strong basicity and reactivity stem from the ionic nature of the Ca-O bond and the relatively small size and high charge density of the Ca $^{2+}$ ion. It readily reacts with acidic oxides and water, making it a versatile industrial chemical.

Key Principles/Laws:

Thermal Decomposition: — The primary method of producing quicklime involves the thermal decomposition of calcium carbonate, an endothermic process governed by Le Chatelier's principle, where high temperatures favor product formation.

Acid-Base Reactions: — Quicklime is a strong base and reacts with acids and acidic oxides to form salts and water, or just salts, respectively.

Hydration (Slaking): — Its reaction with water is a classic example of a hydration reaction, which is highly exothermic.

Derivations (Preparation):

Industrially, calcium oxide is prepared by the thermal decomposition of calcium carbonate (limestone) in a lime kiln at very high temperatures (around $1000-1200^\circ\text{C}$ ). This process is called calcination.

\text{CaCO}_3(\text{s}) \xrightarrow{\text{Heat}} \text{CaO}(\text{s}) + \text{CO}_2(\text{g})

This reaction is reversible, and to ensure a high yield of CaO, carbon dioxide is continuously removed from the kiln, shifting the equilibrium to the right.

Physical Properties:

White, amorphous solid (though can be crystalline).

High melting point (

2572^\circ\text{C}

Highly caustic and corrosive.

Chemical Properties:

Reaction with Water (Slaking of Lime): — Quicklime reacts vigorously with water to form calcium hydroxide (slaked lime). This reaction is highly exothermic, releasing a significant amount of heat.

\text{CaO}(\text{s}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{Ca(OH)}_2(\text{aq}) + \text{Heat}

The process of adding water to quicklime is called slaking of lime.

Reaction with Carbon Dioxide: — Quicklime absorbs carbon dioxide from the air to form calcium carbonate. This is why quicklime must be stored in airtight containers.

\text{CaO}(\text{s}) + \text{CO}_2(\text{g}) \rightarrow \text{CaCO}_3(\text{s})

Reaction with Acids: — Being a basic oxide, quicklime reacts with acids to form calcium salts and water.

\text{CaO}(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{H}_2\text{O}(\text{l})

\text{CaO}(\text{s}) + \text{SO}_2(\text{g}) \rightarrow \text{CaSO}_3(\text{s}) \quad (\text{reaction with acidic oxide})

Real-World Applications:

Cement Industry: — A major component in the manufacture of Portland cement.

Metallurgy: — Used as a flux in the extraction of iron and steel to remove acidic impurities (like silica) from the ore.

Agriculture: — Used to neutralize acidic soils (liming).

Drying Agent: — Due to its strong affinity for water, it's used as a drying agent for gases and alcohol.

Sugar Refining: — Used in the purification of sugar.

II. Calcium Carbonate (CaCO$_3$)

Conceptual Foundation: Calcium carbonate is a salt of calcium and carbonic acid. It is one of the most abundant minerals on Earth, forming vast geological deposits. Its stability and insolubility in pure water (though slightly soluble in water containing dissolved CO $_2$ ) are key to its geological and biological roles.

Key Principles/Laws:

Solubility Equilibrium: — Its solubility in water is influenced by the presence of dissolved carbon dioxide, leading to the formation of soluble calcium bicarbonate, which is crucial in the formation of stalactites and stalagmites.

Thermal Decomposition: — As discussed, it decomposes at high temperatures.

Derivations (Preparation):

Calcium carbonate can be prepared in the laboratory by passing carbon dioxide gas through slaked lime (calcium hydroxide solution).

\text{Ca(OH)}_2(\text{aq}) + \text{CO}_2(\text{g}) \rightarrow \text{CaCO}_3(\text{s}) \downarrow + \text{H}_2\text{O}(\text{l})

If excess carbon dioxide is passed, the insoluble calcium carbonate converts into soluble calcium bicarbonate, which is responsible for temporary hardness of water.

Physical Properties:

White, crystalline solid.

Insoluble in water (practically).

Exists in various crystalline forms, notably calcite (in limestone, marble) and aragonite.

Chemical Properties:

Thermal Decomposition: — Decomposes upon heating to form calcium oxide and carbon dioxide.

\text{CaCO}_3(\text{s}) \xrightarrow{\text{Heat}} \text{CaO}(\text{s}) + \text{CO}_2(\text{g})

Reaction with Acids: — Reacts with dilute acids to produce carbon dioxide gas, a common test for carbonates.

\text{CaCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})

Real-World Applications:

Building Material: — Used extensively as limestone, marble, and chalk in construction.

Raw Material: — For the manufacture of quicklime, cement, and glass.

Antacid: — Used in medicine to neutralize excess stomach acid.

Filler: — In paper, plastics, paints, and rubber industries.

Abrasive: — In toothpaste.

III. Plaster of Paris (CaSO$_4 \cdot \frac{1}{2}$H$_2$O)

Conceptual Foundation: Plaster of Paris, or calcium sulfate hemihydrate, is a unique compound known for its ability to 'set' when mixed with water. This setting property is due to its rehydration to form gypsum, which then crystallizes into an interlocking network, imparting strength and rigidity.

Key Principles/Laws:

Hydration and Crystallization: — The setting process is a rehydration reaction followed by crystallization, forming a stable dihydrate structure.

Controlled Heating: — Its preparation requires precise temperature control to avoid complete dehydration.

Derivations (Preparation):

Plaster of Paris is prepared by heating gypsum (calcium sulfate dihydrate, CaSO $_4 \cdot 2$ H $_2$ O) to a temperature of $100^\circ\text{C}$ ( $373\,\text{K}$ ). At this temperature, gypsum loses three-quarters of its water of crystallization.

\text{CaSO}_4 \cdot 2\text{H}_2\text{O}(\text{s}) \xrightarrow{100^\circ\text{C}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}(\text{s}) + 1\frac{1}{2}\text{H}_2\text{O}(\text{l})

Crucial Note: If gypsum is heated above

100^\circ\text{C}

(e.

g., to $200^\circ\text{C}$ ), it loses all its water of crystallization to form anhydrous calcium sulfate, CaSO $_4$ , which is known as dead burnt plaster. Dead burnt plaster does not have the property of setting with water because its crystalline structure is destroyed, and it cannot rehydrate to form gypsum.

This is a common NEET trap point.

Physical Properties:

White powder.

Low solubility in water.

Chemical Properties:

Setting with Water: — When mixed with an appropriate amount of water, Plaster of Paris rehydrates and sets into a hard, solid mass (gypsum) within 5-15 minutes. This is an exothermic process.

\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}(\text{s}) + 1\frac{1}{2}\text{H}_2\text{O}(\text{l}) \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}(\text{s})

The setting involves the formation of a network of tiny gypsum crystals, which interlock and solidify.

Real-World Applications:

Medical Field: — Used for setting fractured bones, dental impressions, and surgical casts.

Construction: — Used for making decorative elements, false ceilings, and as a fireproofing material.

Art and Sculpture: — For making statues, models, and molds.

Laboratory: — For sealing apparatus and making temporary supports.

Common Misconceptions:

Quicklime vs. Slaked Lime: — Students often confuse CaO (quicklime) with Ca(OH)

_2

(slaked lime). Quicklime is the oxide, while slaked lime is the hydroxide formed by reacting quicklime with water.

Gypsum vs. Plaster of Paris vs. Dead Burnt Plaster: — It's vital to distinguish between these three forms of calcium sulfate based on their water of crystallization and setting properties. Gypsum is the dihydrate, PoP is the hemihydrate, and dead burnt plaster is the anhydrous form. Only PoP effectively sets with water.

Reversibility of Reactions: — While the decomposition of CaCO

_3

is reversible, the setting of PoP is practically irreversible in terms of its structural integrity once set.

NEET-Specific Angle:

NEET questions frequently test the preparation methods, specific reaction conditions (e.g., temperature for PoP formation), chemical formulas, common names, and the unique properties (like the setting of PoP or the basicity of CaO) and their applications. Understanding the stoichiometry of water molecules in gypsum and Plaster of Paris is a common numerical/conceptual trap. Questions on the effect of overheating gypsum (dead burnt plaster) are also common.

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.

Questions students ask

6 answered on this topic.

What is the difference between quicklime and slaked lime?

Quicklime is calcium oxide (CaO), a highly reactive, anhydrous basic oxide. It's produced by heating limestone. Slaked lime, on the other hand, is calcium hydroxide (Ca(OH) $_2$ ), which is formed when quicklime reacts with water. This reaction, known as slaking, is highly exothermic. Quicklime is used as a drying agent and in cement, while slaked lime is used in whitewash and as a flocculant in water treatment. Their chemical formulas and reactivity with water are the key distinguishing factors.

Why is Plaster of Paris called 'Plaster of Paris'?

The name 'Plaster of Paris' originates from the fact that extensive gypsum deposits, the raw material for Plaster of Paris, were historically found in Montmartre, Paris, France. The gypsum from these quarries was widely used to make plaster for building construction and decorative purposes in the city. Thus, the name became associated with its primary source and application, even though it's now produced globally.

What happens if gypsum is heated above $100^\circ\text{C}$?

If gypsum (CaSO $_4 \cdot 2$ H $_2$ O) is heated above $100^\circ\text{C}$ (specifically around $200^\circ\text{C}$ or higher), it loses all its water of crystallization. This results in the formation of anhydrous calcium sulfate (CaSO $_4$ ), which is known as 'dead burnt plaster'. Unlike Plaster of Paris, dead burnt plaster loses its ability to set with water because its crystalline structure is irreversibly destroyed, preventing proper rehydration and interlocking crystal formation.

How does Plaster of Paris set when mixed with water?

When Plaster of Paris (calcium sulfate hemihydrate, CaSO $_4 \cdot \frac{1}{2}$ H $_2$ O) is mixed with water, it rehydrates to form calcium sulfate dihydrate (gypsum, CaSO $_4 \cdot 2$ H $_2$ O). This rehydration process leads to the formation of numerous tiny, needle-like gypsum crystals. These crystals grow and interlock with each other, forming a rigid, solid, and strong crystalline matrix. This interlocking network is what gives the set plaster its hardness and structural integrity.

What are the main industrial uses of calcium carbonate?

Calcium carbonate (CaCO $_3$ ) has numerous industrial applications. It is primarily used as a raw material for the production of quicklime (CaO) and cement. In construction, it's used directly as limestone and marble. It also serves as a filler in the paper, plastics, and paint industries, improving product properties. Furthermore, it's used as an antacid in pharmaceuticals, an abrasive in toothpaste, and as a dietary calcium supplement.

Why is quicklime stored in airtight containers?

Quicklime (CaO) is a highly hygroscopic substance, meaning it readily absorbs moisture from the atmosphere. More importantly, it reacts with carbon dioxide present in the air to form calcium carbonate (CaCO $_3$ ). This reaction, known as carbonation, degrades the quicklime, reducing its purity and effectiveness for its intended applications. Storing it in airtight containers prevents contact with both moisture and carbon dioxide, preserving its chemical integrity.