Solubility Equilibrium
Understanding the balance between dissolution and precipitation of solids in solution
What is Solubility Equilibrium?
Solubility equilibrium is the dynamic balance established when a solid solute dissolves in a solvent until saturation is reached. At this point, the rate of dissolution equals the rate of precipitation, and the concentration of dissolved ions remains constant.
Solubility equilibrium is a dynamic equilibrium established between a solid solute and its dissolved ions in a saturated solution.
General format: AB(s)⇌A+(aq)+B−(aq)\text{AB(s)} \rightleftharpoons \text{A}^+ (aq) + \text{B}^- (aq)AB(s)⇌A+(aq)+B−(aq)
Examples of Solubility Equilibrium
1. Silver Chloride (AgCl)
AgCl (s)⇌Ag+(aq)+Cl−(aq)\text{AgCl (s)} \rightleftharpoons \text{Ag}^+ (aq) + \text{Cl}^- (aq)AgCl (s)⇌Ag+(aq)+Cl−(aq)
- Explanation: In water, only a small amount of AgCl dissolves. The system reaches equilibrium where the rate of dissolution equals the rate of precipitation.
- Ksp (Solubility product) = 1.8×10−101.8 \times 10^{-10}1.8×10−10
2. Calcium Fluoride (CaF₂)
CaF2(s)⇌Ca2+(aq)+2F−(aq)\text{CaF}_2 (s) \rightleftharpoons \text{Ca}^{2+} (aq) + 2\text{F}^- (aq)CaF2(s)⇌Ca2+(aq)+2F−(aq)
- Explanation: For each mole of CaF₂ that dissolves, 1 mole of Ca²⁺ and 2 moles of F⁻ ions are formed.
- Ksp = 3.9×10−113.9 \times 10^{-11}3.9×10−11
3. Barium Sulfate (BaSO₄)
BaSO4(s)⇌Ba2+(aq)+SO42−(aq)\text{BaSO}_4 (s) \rightleftharpoons \text{Ba}^{2+} (aq) + \text{SO}_4^{2-} (aq)BaSO4(s)⇌Ba2+(aq)+SO42−(aq)
- Use: Medical imaging (Barium meals).
- Ksp = 1.1×10−101.1 \times 10^{-10}1.1×10−10
4. Lead(II) Iodide (PbI₂)
PbI2(s)⇌Pb2+(aq)+2I−(aq)\text{PbI}_2 (s) \rightleftharpoons \text{Pb}^{2+} (aq) + 2\text{I}^- (aq)PbI2(s)⇌Pb2+(aq)+2I−(aq)
- Ksp = 7.1×10−97.1 \times 10^{-9}7.1×10−9
5. Iron(III) Hydroxide (Fe(OH)₃)
Fe(OH)3(s)⇌Fe3+(aq)+3OH−(aq)\text{Fe(OH)}_3 (s) \rightleftharpoons \text{Fe}^{3+} (aq) + 3\text{OH}^- (aq)Fe(OH)3(s)⇌Fe3+(aq)+3OH−(aq)
- Low solubility in water.
- Ksp = 6.3×10−386.3 \times 10^{-38}6.3×10−38
6. Magnesium Hydroxide (Mg(OH)₂)
Mg(OH)2(s)⇌Mg2+(aq)+2OH−(aq)\text{Mg(OH)}_2 (s) \rightleftharpoons \text{Mg}^{2+} (aq) + 2\text{OH}^- (aq)Mg(OH)2(s)⇌Mg2+(aq)+2OH−(aq)
- Used as an antacid.
- Ksp = 5.6×10−125.6 \times 10^{-12}5.6×10−12
7. Zinc Hydroxide (Zn(OH)₂)
Zn(OH)2(s)⇌Zn2+(aq)+2OH−(aq)\text{Zn(OH)}_2 (s) \rightleftharpoons \text{Zn}^{2+} (aq) + 2\text{OH}^- (aq)Zn(OH)2(s)⇌Zn2+(aq)+2OH−(aq)
- Amphoteric behavior: Dissolves in both acid and base.
- Ksp = 4.5×10−174.5 \times 10^{-17}4.5×10−17
Common Conceptual Questions Based on Solubility Equilibrium:
- How does common ion effect reduce solubility?
- How to calculate solubility from Ksp?
- How does pH affect solubility of hydroxide salts?
Quick Summary Table
| Salt | Dissociation Equation | Ksp (approx.) |
|---|---|---|
| AgCl | AgCl ⇌ Ag⁺ + Cl⁻ | 1.8×10−101.8 × 10^{-10}1.8×10−10 |
| CaF₂ | CaF₂ ⇌ Ca²⁺ + 2F⁻ | 3.9×10−113.9 × 10^{-11}3.9×10−11 |
| BaSO₄ | BaSO₄ ⇌ Ba²⁺ + SO₄²⁻ | 1.1×10−101.1 × 10^{-10}1.1×10−10 |
| PbI₂ | PbI₂ ⇌ Pb²⁺ + 2I⁻ | 7.1×10−97.1 × 10^{-9}7.1×10−9 |
| Fe(OH)₃ | Fe(OH)₃ ⇌ Fe³⁺ + 3OH⁻ | 6.3×10−386.3 × 10^{-38}6.3×10−38 |
| Mg(OH)₂ | Mg(OH)₂ ⇌ Mg²⁺ + 2OH⁻ | 5.6×10−125.6 × 10^{-12}5.6×10−12 |
| Zn(OH)₂ | Zn(OH)₂ ⇌ Zn²⁺ + 2OH⁻ | 4.5×10−174.5 × 10^{-17}4.5×10−17 |
Dynamic Nature of Solubility Equilibrium
This is a reversible process:
Solid (s) ↔ Dissolved ions (aq)
At equilibrium, both dissolution and precipitation occur simultaneously with no net change in concentrations.
Solubility Product Constant (Ksp)
For a salt AB dissociating in water:
AB(s) ↔ A+(aq) + B–(aq)
The solubility product is given as:
Ksp = [A+][B–]
Smaller Ksp means the substance is less soluble in water.
Factors Affecting Solubility Equilibrium
- Common Ion Effect: Presence of a common ion reduces solubility by shifting equilibrium to the left.
- pH of Solution: Salts with acidic/basic components dissolve more/less in acidic/basic environments.
- Temperature: Generally, solubility increases with temperature (except for some salts).
Ionic Product and Precipitation
The Ionic Product (IP) is the product of ion concentrations at any moment:
- If IP < Ksp: more solid will dissolve.
- If IP = Ksp: solution is saturated, at equilibrium.
- If IP > Ksp: precipitation occurs.
Applications of Solubility Equilibrium
- Predicting and controlling precipitation reactions.
- Water purification and softening.
- Biomineralization (e.g., bone and teeth formation).
- Preventing industrial scaling in boilers and pipes.
Mastering solubility equilibrium is essential in chemistry, medicine, and engineering.
MCQs: Solubility Equilibrium
- What does Ksp represent?
a) Solubility
b) Ionic product
c) Solubility product constant ✅
d) Precipitation rate
Explanation: Ksp is the equilibrium constant for the dissociation of a salt in water. - What happens when IP > Ksp?
a) More solid dissolves
b) Precipitation occurs ✅
c) Equilibrium is reached
d) No change
Explanation: If IP exceeds Ksp, excess ions combine to form solid precipitate.
True/False Questions
- Solubility equilibrium is a static condition.
❌ False – It is dynamic, with equal rates of dissolution and precipitation. - Common ion effect increases solubility.
❌ False – It actually decreases solubility.
Quick Quiz
Fill in the blank: When the ionic product equals the solubility product constant, the solution is ________.
Answer: Saturated
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