Calcium Oxide, Calcium Carbonate, Plaster of Paris — Revision Notes

⚡ 30-Second Revision

Quicklime: — CaO, Calcium Oxide. Basic oxide. From CaCO$_3$ (heat). Reacts with H$_2$O (slaking, exothermic) to Ca(OH)$_2$.
Calcium Carbonate: — CaCO$_3$. Limestone, marble, chalk. Decomposes to CaO + CO$_2$ (heat). Reacts with acids to CO$_2$.
Gypsum: — CaSO$_4 cdot 2$H$_2$O. Raw material for PoP.
Plaster of Paris (PoP): — CaSO$_4 cdot \frac{1}{2}$H$_2$O. Calcium Sulfate Hemihydrate. From gypsum (heat to $100^circ\text{C}$). Sets with H$_2$O to CaSO$_4 cdot 2$H$_2$O (gypsum).
Dead Burnt Plaster: — CaSO$_4$. Anhydrous. From gypsum (heat > $200^circ\text{C}$). Loses setting property.

2-Minute Revision

For NEET, remember these key facts about calcium compounds. Calcium Oxide (CaO), or quicklime, is a strong basic oxide made by heating limestone (CaCO$_3$) to high temperatures. It's highly reactive, especially with water, forming **Calcium Hydroxide (Ca(OH)$_2$)** or slaked lime in an exothermic reaction.

Quicklime is vital in cement and as a drying agent. **Calcium Carbonate (CaCO$_3$)** is abundant as limestone, marble, and chalk. It decomposes to quicklime and CO$_2$ upon heating and reacts with acids to release CO$_2$.

It's used in construction and as an antacid. **Plaster of Paris (CaSO$_4 cdot \frac{1}{2}$H$_2$O) is calcium sulfate hemihydrate. It's prepared by heating Gypsum (CaSO$_4 cdot 2$H$_2$O)** to precisely $100^circ\text{C}$.

Its unique property is setting into a hard mass (gypsum) when mixed with water, making it indispensable for surgical casts and molds. Crucially, overheating gypsum above $200^circ\text{C}$ yields **dead burnt plaster (anhydrous CaSO$_4$)**, which loses its setting ability due to irreversible structural changes.

Always pay attention to chemical formulas, reaction conditions, and applications.

5-Minute Revision

Let's consolidate the crucial aspects of Calcium Oxide, Calcium Carbonate, and Plaster of Paris for NEET. Calcium Oxide (CaO), known as quicklime, is a white, basic, amorphous solid. Its industrial preparation involves the thermal decomposition of calcium carbonate (limestone) at $1000-1200^circ\text{C}$: $ext{CaCO}_3(\text{s}) xrightarrow{\text{Heat}} \text{CaO}(\text{s}) + \text{CO}_2(\text{g})$.

Quicklime reacts vigorously with water (slaking) to form calcium hydroxide (slaked lime), $ext{Ca(OH)}_2$: $ext{CaO}(\text{s}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{Ca(OH)}_2(\text{aq})$. This reaction is highly exothermic.

Quicklime is used in cement, as a metallurgical flux, and as a drying agent.

**Calcium Carbonate (CaCO$_3$)** is found naturally as limestone, marble, and chalk. It's prepared in the lab by passing CO$_2$ through slaked lime: $ext{Ca(OH)}_2(\text{aq}) + \text{CO}_2(\text{g}) \rightarrow \text{CaCO}_3(\text{s}) + \text{H}_2\text{O}(\text{l})$.

If excess CO$_2$ is passed, the precipitate dissolves to form soluble calcium bicarbonate, $ext{Ca(HCO}_3)_2$. CaCO$_3$ reacts with acids to liberate CO$_2$: $ext{CaCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})$.

Its uses include building materials, antacids, and as a filler.

Plaster of Paris (PoP), or calcium sulfate hemihydrate ($ext{CaSO}_4 cdot \frac{1}{2}\text{H}_2\text{O}$), is derived from **Gypsum ($ext{CaSO}_4 cdot 2\text{H}_2\text{O}$)** by heating it to $100^circ\text{C}$ ($373,\text{K}$): $ext{CaSO}_4 cdot 2\text{H}_2\text{O}(\text{s}) xrightarrow{100^circ\text{C}} \text{CaSO}_4 cdot \frac{1}{2}\text{H}_2\text{O}(\text{s}) + 1\frac{1}{2}\text{H}_2\text{O}(\text{l})$.

The critical property of PoP is its ability to set into a hard mass when mixed with water, rehydrating to form gypsum: $ext{CaSO}_4 cdot \frac{1}{2}\text{H}_2\text{O}(\text{s}) + 1\frac{1}{2}\text{H}_2\text{O}(\text{l}) \rightarrow \text{CaSO}_4 cdot 2\text{H}_2\text{O}(\text{s})$.

This makes it invaluable for surgical casts, dental impressions, and molds. Remember, overheating gypsum above $200^circ\text{C}$ produces **dead burnt plaster (anhydrous $ext{CaSO}_4$)**, which loses its setting property due to irreversible structural changes.

Pay close attention to the water of crystallization and the specific temperatures involved in these conversions.

Prelims Revision Notes

1
Calcium Oxide (Quicklime), CaO:

* Preparation: Thermal decomposition of CaCO$_3$ (limestone) at $1000-1200^circ\text{C}$ in a lime kiln. $ext{CaCO}_3(\text{s}) xrightarrow{\text{Heat}} \text{CaO}(\text{s}) + \text{CO}_2(\text{g})$. * Properties: White, amorphous, basic oxide.

High melting point ($2572^circ\text{C}$). Highly reactive. * Reactions: * Slaking of lime: With water, forms calcium hydroxide (slaked lime). Exothermic. $ext{CaO}(\text{s}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{Ca(OH)}_2(\text{aq})$.

* With CO$_2$: Forms CaCO$_3$. $ext{CaO}(\text{s}) + \text{CO}_2(\text{g}) \rightarrow \text{CaCO}_3(\text{s})$. * With acids: Forms salt + water. $ext{CaO}(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{H}_2\text{O}(\text{l})$.

* Uses: Cement, metallurgy (flux), drying agent, agriculture (liming).

1
**Calcium Carbonate (Limestone, Marble, Chalk), CaCO$_3$:**

* Occurrence: Abundant in nature. * Preparation (lab): Passing CO$_2$ through slaked lime. $ext{Ca(OH)}_2(\text{aq}) + \text{CO}_2(\text{g}) \rightarrow \text{CaCO}_3(\text{s}) downarrow + \text{H}_2\text{O}(\text{l})$.

* Properties: White, insoluble solid. * Reactions: * Thermal decomposition: To CaO + CO$_2$ (reverse of quicklime prep). $ext{CaCO}_3(\text{s}) xrightarrow{\text{Heat}} \text{CaO}(\text{s}) + \text{CO}_2(\text{g})$.

* With acids: Liberates CO$_2$. $ext{CaCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})$. * With excess CO$_2$ + H$_2$O: Forms soluble calcium bicarbonate ($ext{Ca(HCO}_3)_2$).

* Uses: Building material, antacid, raw material for cement/glass, filler.

1
**Plaster of Paris (PoP), CaSO$_4 cdot \frac{1}{2}$H$_2$O:**

* Raw material: Gypsum ($ext{CaSO}_4 cdot 2$H$_2$O). * Preparation: Heating gypsum to $100^circ\text{C}$ ($373,\text{K}$). $ext{CaSO}_4 cdot 2\text{H}_2\text{O}(\text{s}) xrightarrow{100^circ\text{C}} \text{CaSO}_4 cdot \frac{1}{2}\text{H}_2\text{O}(\text{s}) + 1\frac{1}{2}\text{H}_2\text{O}(\text{l})$.

* Properties: White powder. * Key Reaction (Setting): Rehydrates with water to form gypsum, setting into a hard mass. $ext{CaSO}_4 cdot \frac{1}{2}\text{H}_2\text{O}(\text{s}) + 1\frac{1}{2}\text{H}_2\text{O}(\text{l}) \rightarrow \text{CaSO}_4 cdot 2\text{H}_2\text{O}(\text{s})$.

Exothermic. * Dead Burnt Plaster: Anhydrous CaSO$_4$. Formed by heating gypsum above $200^circ\text{C}$. Loses setting property due to irreversible structural damage. * Uses: Surgical casts, dental impressions, molds, decorative elements, fireproofing.

Vyyuha Quick Recall

To remember the calcium compounds and their water content: Great People Drink Water Gypsum: 2 H$_2$O (CaSO$_4 cdot 2$H$_2$O) Plaster of Paris: 1/2 H$_2$O (CaSO$_4 cdot \frac{1}{2}$H$_2$O) Dead burnt plaster: 0 H$_2$O (CaSO$_4$) This helps recall the decreasing water content as gypsum is heated.