Nature of Solutions
High-Yield Summary
- Water is a bent, polar molecule (H–O–H ≈ 104.5°) — hydrogen-bonds with itself and hydrates dissolved ions (positive H ends toward anions, negative O end toward cations), making it an effective solvent.
- Solute dissolves in solvent; solvation (called hydration in water) replaces solute-solute and solvent-solvent interactions with solute-solvent interactions — exothermic (favored at low T), endothermic (favored at high T), or near-zero enthalpy (ideal solution).
- Solubility is the max dissolvable solute at given T/P. Solutions are unsaturated (below limit), saturated (at limit, dynamic equilibrium), or supersaturated (metastable, above normal limit).
- Strong electrolytes fully dissociate into ions (NaCl, KOH, HCl, H₂SO₄); weak electrolytes partially dissociate (CH₃COOH, NH₃); nonelectrolytes don't dissociate at all (sucrose, ethanol).
- Seven solubility rules predict whether an ionic compound dissolves; complex ions (ligands donating electron pairs to a central metal ion) can increase the solubility of an otherwise-insoluble compound.
Key Terms
- Solvation
- The electrostatic interaction between solute and solvent molecules; called hydration when the solvent is water.
- Solubility
- The maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature and pressure.
- Saturated solution
- Holds the max solute possible at that T/P; dynamic equilibrium where dissolution rate equals crystallization rate.
- Supersaturated solution
- Holds more dissolved solute than it normally could — metastable, achieved by dissolving at high T then carefully cooling.
- Complex ion
- A central metal ion bonded to one or more ligands via coordinate covalent bonds (ligand supplies both electrons).
Electrolyte Types
| Type | Behavior |
|---|---|
| Strong electrolyte | Fully dissociates into ions — efficient conductor. Ex: NaCl, KOH, HCl, H₂SO₄. |
| Weak electrolyte | Partially dissociates (ionization equilibrium) — low conductivity. Ex: CH₃COOH, NH₃. |
| Nonelectrolyte | Doesn't dissociate at all — no conductivity even if highly soluble. Ex: sucrose, ethanol. |
The Seven Solubility Rules
| Soluble (normally) | Exceptions (insoluble) |
|---|---|
| Alkali metal (Group 1) and NH₄⁺ salts | None |
| Nitrate (NO₃⁻) and acetate (CH₃COO⁻) salts | None |
| Halides (Cl⁻, Br⁻, I⁻), excluding fluorides | Ag⁺, Pb²⁺, Hg₂²⁺ |
| Sulfates (SO₄²⁻) | Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺ |
| Metal oxides: alkali metal, CaO, SrO, BaO | All others insoluble |
| Hydroxides: alkali metal, NH₄⁺, Ca²⁺, Sr²⁺, Ba²⁺ | All others insoluble |
| Carbonates/phosphates/sulfides/sulfites: alkali metal, NH₄⁺ | All others insoluble |
Must-Know Points
- Water's H bonds are covalent, but O is more electronegative — pulls bonding electrons toward itself, giving O partial negative charge and each H partial positive charge.
- Complex ion formation can pull an otherwise-insoluble salt into solution — e.g., excess NH₃ forms soluble [Ag(NH₃)₂]⁺ from insoluble AgCl. Hemoglobin's heme group chelating iron is a biological example.
- Solubility of ionic compounds depends on the balance between ion-ion attraction in the solid and ion-water attraction — not a single fixed rule per compound.
Common MCAT Trap
- (NH₄)₂CO₃ is soluble even though carbonates are normally insoluble — the ammonium exception overrides the anion rule. Always check both the anion rule AND the cation exceptions.
- PbSO₄ is insoluble even though sulfates are normally soluble — Pb²⁺ is a named exception. Don't stop at the anion category.
- A supersaturated solution is NOT at equilibrium — it's metastable and will crystallize rapidly if disturbed or seeded, unlike a true saturated (equilibrium) solution.
Quick Recall
Is KOH soluble in water?
What's the difference between solvation and hydration?
Why does adding excess NH₃ dissolve insoluble AgCl?