Important Chemical Compounds of Calcium (Ca)

Introduction to the Major Compounds of Calcium

Calcium (Ca) is an alkaline earth metal, group 2 element, widely distributed in nature as silicates, carbonates, and sulfates. Its compounds are crucial in industry, construction, and biology. This guide covers the most important calcium compounds relevant for high school examinations.

1. Calcium Oxide (CaO)

Chemical Formula and Common Name

• Chemical Formula: CaO
• Common Name: Quicklime, Burnt lime

Laboratory Preparation

Calcium oxide is prepared on an industrial scale by the thermal decomposition of limestone (calcium carbonate).

• Process: Limestone is heated to a high temperature (1000-1200°C) in a rotary kiln.
• Balanced Equation:

  CaCO₃(s)  $\xrightarrow{\text{1000-1200°C}}$  CaO(s) + CO₂(g)

Properties and Exam-Relevant Reactions

1. Nature: It is a white, amorphous solid with a high melting point (2870 K).
2. Reaction with Water (Slaking of Lime): CaO reacts vigorously with water, releasing a large amount of heat to form calcium hydroxide.

   CaO(s) + H₂O(l)  →  Ca(OH)₂(s) + Heat

3. Basic Oxide Nature: Being a basic oxide, it reacts with acidic oxides.

   CaO(s) + SiO₂(s)  $\xrightarrow{\Delta}$  CaSiO₃(s)

   CaO(s) + SO₂(g)  →  CaSO₃(s)

4. Uses: In cement and mortar, as a flux in metallurgy, for removing acidity from soil, and as a drying agent.

2. Calcium Hydroxide (Ca(OH)₂)

Chemical Formula and Common Name

• Chemical Formula: Ca(OH)₂
• Common Name: Slaked lime, Hydrated lime
• Aqueous Solution: Limewater (clear solution)
• Suspension in Water: Milk of lime (milky suspension)

Preparation Process

Calcium hydroxide is primarily prepared by the ‘slaking’ of quicklime with water.

• Process: Water is slowly added to quicklime (CaO). The reaction is exothermic.
• Balanced Equation:

  CaO(s) + H₂O(l)  →  Ca(OH)₂(s)

Properties and Exam-Relevant Reactions

1. Nature: White amorphous powder, sparingly soluble in water.
2. Reaction with Carbon Dioxide:
   • Milky appearance (Test for CO₂): Limewater turns milky when CO₂ is passed through it due to the formation of insoluble calcium carbonate.

     Ca(OH)₂(aq) + CO₂(g)  →  CaCO₃(s)↓ + H₂O(l)

   • Formation of soluble bicarbonate: If excess CO₂ is passed, the milky precipitate disappears as calcium bicarbonate (soluble) is formed.

     CaCO₃(s) + CO₂(g) + H₂O(l)  →  Ca(HCO₃)₂(aq)

3. Reaction with Chlorine: Forms bleaching powder (calcium oxychloride).

   Ca(OH)₂(s) + Cl₂(g)  →  CaOCl₂(s) + H₂O(l)

4. Uses: In mortar, whitewash, preparation of bleaching powder, as an antacid, and in sugar refining.

3. Other Important Calcium Compounds

A. Calcium Carbonate (CaCO₃)

• Chemical Formula: CaCO₃
• Common Names: Limestone, Marble, Chalk, Dolomite (CaCO₃·MgCO₃)

Preparation

• From Calcium Hydroxide: Prepared by passing carbon dioxide through limewater.

  Ca(OH)₂(aq) + CO₂(g)  →  CaCO₃(s)↓ + H₂O(l)

• From Calcium Chloride (Industrial):

  CaCl₂(aq) + Na₂CO₃(aq)  →  CaCO₃(s)↓ + 2NaCl(aq)

Properties and Exam-Relevant Reactions

1. Nature: White solid, insoluble in water (but soluble in water containing CO₂ due to bicarbonate formation).
2. Thermal Decomposition: Decomposes upon heating to form quicklime and carbon dioxide.

   CaCO₃(s)  $\xrightarrow{\text{1000-1200°C}}$  CaO(s) + CO₂(g)

3. Reaction with Acids: Reacts with dilute acids to produce carbon dioxide gas.

   CaCO₃(s) + 2HCl(aq)  →  CaCl₂(aq) + H₂O(l) + CO₂(g)↑

   CaCO₃(s) + H₂SO₄(aq)  →  CaSO₄(s) + H₂O(l) + CO₂(g)↑

4. Uses: Building material (marble, limestone), as an antacid, in toothpaste, as a filler in rubber, and in the manufacture of cement.

B. Calcium Sulfate Hemihydrate (CaSO₄·½H₂O)

• Chemical Formula: CaSO₄·½H₂O
• Common Name: Plaster of Paris (POP)
• Related Compound: Gypsum (CaSO₄·2H₂O)

Preparation

• From Gypsum: Plaster of Paris is obtained by heating gypsum (calcium sulfate dihydrate) to a specific temperature (373 K or 100°C).
• Process: Heating gypsum carefully ensures that only three-quarters of the water of crystallization is lost. Overheating leads to anhydrous calcium sulfate (dead burnt plaster), which does not set with water.
• Balanced Equation:

  CaSO₄·2H₂O(s)  $\xrightarrow{\text{373 K}}$  CaSO₄·½H₂O(s) + 1½H₂O(g)

Properties and Exam-Relevant Reactions

1. Nature: White powder.
2. Setting Property: Its most important property is its ability to set into a hard mass upon mixing with water. This is due to rehydration to form gypsum.

   CaSO₄·½H₂O(s) + 1½H₂O(l)  →  CaSO₄·2H₂O(s)

3. Uses: In surgical bandages for fractured bones, making casts for statues and toys, fire-proofing materials, and as a decorative material for ceilings and walls.

Comparative Properties of Key Calcium Compounds

Compound	Chemical Formula	Common Name	Key Property/Use
Calcium Oxide	CaO	Quicklime	Slaking with water, basic oxide, cement manufacturing
Calcium Hydroxide	Ca(OH)₂	Slaked Lime	Turns milky with CO₂, forms bleaching powder
Calcium Carbonate	CaCO₃	Limestone, Marble	Decomposes on heating, reacts with acids, building material
Calcium Sulfate Hemihydrate	CaSO₄·½H₂O	Plaster of Paris	Sets with water to form gypsum, surgical casts
Calcium Sulfate Dihydrate	CaSO₄·2H₂O	Gypsum	Source for POP, fertilizer