Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate. This comprehensive reference provides all essential formulas related to gypsum and its various forms, designed for students studying chemistry, geology, and materials science.
Main Gypsum Formulas
| Formula Type | Chemical Formula | Common Name | Molecular Weight | Water Content | Description |
|---|---|---|---|---|---|
| Primary Gypsum Formula | CaSO₄·2H₂O | Gypsum (Dihydrate) | 172.17 g/mol | 2 water molecules | Natural hydrated calcium sulfate mineral |
| Anhydrous Gypsum Formula | CaSO₄ | Anhydrite | 136.14 g/mol | 0 water molecules | Completely dehydrated form of gypsum |
| Hemihydrate Formula | CaSO₄·½H₂O | Plaster of Paris (β-form) | 145.15 g/mol | 0.5 water molecules | Partially dehydrated gypsum used in construction |
| Alpha Hemihydrate Formula | CaSO₄·½H₂O | Dental Plaster (α-form) | 145.15 g/mol | 0.5 water molecules | High-strength hemihydrate with different crystal structure |
| Bassanite Formula | CaSO₄·½H₂O | Bassanite | 145.15 g/mol | 0.5 water molecules | Natural hemihydrate mineral |
| Selenite Formula | CaSO₄·2H₂O | Selenite | 172.17 g/mol | 2 water molecules | Transparent crystalline form of gypsum |
| Alabaster Formula | CaSO₄·2H₂O | Alabaster | 172.17 g/mol | 2 water molecules | Fine-grained, massive form of gypsum |
Chemical Reaction Formulas
Dehydration Reactions
| Reaction Type | Chemical Equation | Temperature Range | Application |
|---|---|---|---|
| Partial Dehydration | CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O | 120-180°C | Production of Plaster of Paris |
| Complete Dehydration | CaSO₄·2H₂O → CaSO₄ + 2H₂O | 300-400°C | Production of anhydrite |
| Two-Step Dehydration | CaSO₄·½H₂O → CaSO₄ + ½H₂O | 180-300°C | Complete water removal |
Hydration Reactions
| Reaction Type | Chemical Equation | Process | Time Required |
|---|---|---|---|
| Plaster Setting | CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O | Hardening of plaster | 10-30 minutes |
| Anhydrite Hydration | CaSO₄ + 2H₂O → CaSO₄·2H₂O | Slow natural process | Days to weeks |
Solubility Formulas
| Parameter | Formula/Value | Conditions | Units |
|---|---|---|---|
| Solubility Product (Ksp) | Ksp = [Ca²⁺][SO₄²⁻] | At 25°C | 2.4 × 10⁻⁵ |
| Solubility in Water | 2.6 g/L | At 25°C | grams per liter |
| Solubility Equation | CaSO₄·2H₂O ⇌ Ca²⁺ + SO₄²⁻ + 2H₂O | Equilibrium | – |
Percentage Composition Formulas
| Component | Formula for % Calculation | Percentage in Gypsum |
|---|---|---|
| Calcium (Ca) | (40.08/172.17) × 100 | 23.28% |
| Sulfur (S) | (32.07/172.17) × 100 | 18.62% |
| Oxygen (O) | (112.00/172.17) × 100 | 65.07% |
| Hydrogen (H) | (4.03/172.17) × 100 | 2.34% |
| Water Content | (36.03/172.17) × 100 | 20.93% |
Industrial Application Formulas
| Application | Formula/Calculation | Purpose | Industry |
|---|---|---|---|
| Cement Retarder | 2-5% CaSO₄·2H₂O by weight | Controls setting time | Construction |
| Soil Amendment | pH = 6.5-7.5 (target range) | Improves soil structure | Agriculture |
| Paper Filler | 10-25% CaSO₄·2H₂O by weight | Increases opacity | Paper Manufacturing |
Crystal System and Properties
| Property | Formula/Value | Description |
|---|---|---|
| Crystal System | Monoclinic | a ≠ b ≠ c, α = γ = 90°, β ≠ 90° |
| Density Formula | ρ = m/V | 2.31-2.33 g/cm³ for gypsum |
| Hardness (Mohs) | 1.5-2.0 | Very soft mineral |
Learning Points
Chemical Properties
- Molecular Formula: CaSO₄·2H₂O represents one calcium ion, one sulfate ion, and two water molecules
- Hydration State: The number after H₂O indicates water molecules per formula unit
- Reversible Reactions: Dehydration and hydration processes are generally reversible
Physical Properties
- Solubility: Gypsum has limited solubility in water, making it useful for construction
- Thermal Behavior: Heat treatment changes the hydration state and properties
- Crystal Forms: Different crystal forms have the same chemical formula but different physical properties
Tips for Students
- Remember the Pattern: Natural gypsum always has 2 water molecules (dihydrate)
- Plaster Connection: Plaster of Paris is hemihydrate (½ water molecule)
- Temperature Memory: Higher temperatures remove more water
- Reversibility: Most gypsum reactions can go both ways with proper conditions
Applications in Real Life
- Construction: Drywall, plaster, cement additive
- Medicine: Dental casts, orthopedic casts
- Art: Sculpting material (alabaster)
- Agriculture: Soil conditioner and fertilizer
- Industry: Paper filler, paint extender
FAQs on Gypsum Formulas
Q: What is the chemical formula of gypsum?
The chemical formula of gypsum is CaSO₄·2H₂O (calcium sulfate dihydrate).
Detailed Explanation:
- Ca = Calcium (one atom)
- SO₄ = Sulfate ion (one sulfur atom bonded to four oxygen atoms)
- 2H₂O = Two water molecules (hydrated water)
This means each molecule of gypsum contains one calcium ion, one sulfate group, and two water molecules chemically bound within its crystal structure. The molecular weight is 172.17 g/mol. Gypsum is called a “dihydrate” because of the two water molecules present in its structure.
Why This Matters: The presence of water molecules makes gypsum different from anhydrite (which has no water) and gives it unique properties useful in construction and medicine.
Q: What is the difference between gypsum and Plaster of Paris formula?
- Gypsum Formula: CaSO₄·2H₂O (contains 2 water molecules)
- Plaster of Paris Formula: CaSO₄·½H₂O (contains ½ water molecule)
Detailed Explanation: Plaster of Paris is the hemihydrate form of gypsum, meaning it has half the water content of natural gypsum. The key differences are:
| Property | Gypsum | Plaster of Paris |
|---|---|---|
| Water molecules | 2 | 0.5 |
| Physical state | Stable mineral | Powder that sets when mixed with water |
| Molecular weight | 172.17 g/mol | 145.15 g/mol |
| Common use | Natural mineral, soil amendment | Casting, molding, construction |
Practical Application: When you add water to Plaster of Paris, it absorbs water and converts back to gypsum (CaSO₄·2H₂O), which causes it to harden and set.
Q: How is gypsum converted to Plaster of Paris? Write the chemical equation.
Gypsum is converted to Plaster of Paris by heating it to 120-180°C, which removes 1.5 water molecules.
Chemical Equation:
CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
(Gypsum) Heat (Plaster of Paris) (Water vapor)
120-180°CExplanation:
- Process Name: Calcination or partial dehydration
- Temperature: 120-180°C (248-356°F)
- Water Released: 1.5 molecules (or 75% of total water)
- Product: White powder (hemihydrate)
Reverse Reaction (Setting of Plaster):
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O + Heat
(Plaster of Paris) (Water added) (Hardened gypsum)Why This Is Important: This reversible reaction is the basis for using plaster in construction, art, and medical casts. The setting process is exothermic, releasing heat as the plaster hardens.
Q: What is the molecular mass/weight of gypsum?
The molecular mass of gypsum (CaSO₄·2H₂O) is 172.17 g/mol.
Detailed Calculation:
| Element/Component | Atomic Mass | Number of Atoms | Total Mass |
|---|---|---|---|
| Calcium (Ca) | 40.08 g/mol | 1 | 40.08 g/mol |
| Sulfur (S) | 32.07 g/mol | 1 | 32.07 g/mol |
| Oxygen in SO₄ (O) | 16.00 g/mol | 4 | 64.00 g/mol |
| Water (H₂O) | 18.015 g/mol | 2 | 36.03 g/mol |
| Total Molecular Mass | 172.17 g/mol |
For Comparison:
- Anhydrite (CaSO₄) = 136.14 g/mol
- Plaster of Paris (CaSO₄·½H₂O) = 145.15 g/mol
Practical Use: This molecular mass is essential for stoichiometric calculations in chemistry labs, industrial production planning, and determining water content percentages (water is 20.93% of gypsum’s mass).
Q: What is the difference between gypsum, anhydrite, and bassanite formulas?
These three compounds are different hydration states of calcium sulfate:
| Mineral Name | Chemical Formula | Water Content | Common Name | Key Difference |
|---|---|---|---|---|
| Gypsum | CaSO₄·2H₂O | 2 water molecules | Dihydrate | Natural form, most hydrated |
| Bassanite | CaSO₄·½H₂O | 0.5 water molecules | Hemihydrate | Same as Plaster of Paris |
| Anhydrite | CaSO₄ | 0 water molecules | Anhydrous | Completely dehydrated |
Detailed Explanation:
Gypsum (CaSO₄·2H₂O):
- Most common natural form
- Soft, easily scratched
- Used in drywall and construction
Bassanite (CaSO₄·½H₂O):
- Partially dehydrated form
- Natural mineral form of Plaster of Paris
- Sets quickly when mixed with water
Anhydrite (CaSO₄):
- No water in structure
- Harder than gypsum
- Forms naturally in evaporite deposits
- Very slowly converts to gypsum when exposed to water
Progressive Dehydration Sequence:
CaSO₄·2H₂O → CaSO₄·½H₂O → CaSO₄
(Gypsum) Heat (Bassanite) Heat (Anhydrite)
120-180°C 300-400°CWhy This Matters: Understanding these forms helps in construction (choosing the right material), geology (identifying minerals), and chemistry (predicting reactions).