Solubility Product (Ksp) Calculator
Interactive Ksp calculator with 70+ NCERT salts. Calculate molar solubility, compare salts, and master precipitation for JEE & NEET.
The "Why" Behind Solubility
Why does salt (NaCl) dissolve despite strong ionic bonds?
The Challenge: Strong Ionic Bonds
In a solid crystal like NaCl, Na⁺ and Cl⁻ ions are locked together by powerful electric forces. Breaking this requires huge energy (Lattice Energy). Yet, water breaks it easily. How?
1. The Energy Payback
Water surrounds the ions, releasing Hydration Energy. If this energy is comparable to the lattice energy, the crystal weakens.
2. The Freedom 'Bonus' (Entropy)
Nature blindly favors disorder. Free ions swimming in water have much higher Entropy than a rigid crystal.
This drive for freedom pushes the process forward, even if energy is tight.
TL;DR: Dissolution is a trade-off. We pay energy to break the bond, but we get paid back in Hydration Energy and Freedom (Entropy).
Energetics Proof (NaCl)
Negative ΔG means Spontaneous Dissolving.
Deep Dive: The Hydration Shell
How water molecules attack and stabilize ions
Ion-Dipole Interactions (Solvation)
Cation (+)
Oxygen (δ-) end of water points inward towards the positive ion due to electrostatic attraction.
Anion (−)
Hydrogen (δ+) ends of water point inward towards the negative ion.
2. The Size Effect (Charge Density)
Like a concentrated magnet. Pulls water strongly.
Charge is spread out. Holds water weakly.
💧 Why Small Ions Form Hydrated Salts
Small ions have very high hydration enthalpies due to their concentrated charge. This strong attraction for water means they retain water molecules even in their solid crystal form.
LiCl·H₂O — Lithium chloride is so hygroscopic it's used as a desiccant!
ΔHhyd = −520 kJ/mol
BeSO₄·4H₂O — Extremely high charge density = tightly bound water.
ΔHhyd = −2494 kJ/mol 🔥
MgCl₂·6H₂O, MgSO₄·7H₂O (Epsom Salt) — Forms multiple hydrates.
ΔHhyd = −1920 kJ/mol
💡 JEE/NEET Tip: Down a group, hydration enthalpy decreases (Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺). That's why LiCl is deliquescent but NaCl is not!
Group 2 Solubility Lab
Explore why solubility trends flip for different salts.
Group 2 Elements
decreases SLIGHTLY (Value is dominated by the huge size of anion)
drops SIGNIFICANTLY (Hydration is highly sensitive to cation size growth)
Result: Soluble! 💧
Hydration is currently stronger than Lattice Energy.
Alkali Halide Solubility: The Size Mismatch Rule
A significant mismatch in ionic sizes leads to lower lattice energy and thus higher solubility. See how fluorides and iodides show opposite trends!
Solubility vs Cation Radius
Ion Size Comparator
Select Cation
Select Anion
Li⁺
73 pm
I⁻
206 pm
Large mismatch! The Lithium ion (73 pm) and Iodide ion (206 pm) have very different sizes, resulting in a weaker lattice and higher solubility.
Key Observations
LiI and CsF are highly soluble due to large size mismatch (133pm and 62pm respectively).
LiF and CsI are less soluble because ions have similar sizes (46pm and 25pm difference).
This explains why the fluoride and iodide solubility curves cross — opposite trends based on size matching!
📚 Textbook Application
Standard examples from NCERT (Class 11, S-Block Elements)
Problem 10.4
"Why does the solubility of alkaline earth metal hydroxides in water increase down the group?"
Solution
Among alkaline earth metal hydroxides, the anion being common the cationic radius will influence the lattice enthalpy. Since lattice enthalpy decreases much more than the hydration enthalpy with increasing ionic size, the solubility increases as we go down the group.
Problem 10.5
"Why does the solubility of alkaline earth metal carbonates and sulphates in water decrease down the group?"
Solution
The size of anions being much larger compared to cations, the lattice enthalpy will remain almost constant within a particular group. Since the hydration enthalpies decrease down the group, solubility will decrease as found for alkaline earth metal carbonates and sulphates.
📋Solubility Rules
Common patterns in water at 25°C
Always Soluble
Alkali Metals & Ammonium
Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺
Nitrates & Acetates
NO₃⁻, CH₃COO⁻, ClO₃⁻
Halides (Cl, Br, I)
Most are soluble
Sulfates (SO₄²⁻)
Most are soluble
Generally Insoluble
Hydroxides (OH⁻)
Insoluble except Rule #1
Carbonates & Phosphates
CO₃²⁻, PO₄³⁻, S²⁻, CrO₄²⁻
Sulfides (S²⁻)
Most transition metal sulfides