General Chemistry
Solubility
How much of one substance dissolves in another — like dissolves like
Solubility is the maximum amount of solute that dissolves in a given solvent at given temperature. Driven by intermolecular forces — "like dissolves like." Polar solvents (water) dissolve polar/ionic solutes; nonpolar solvents (hexane) dissolve nonpolar solutes. Quantified: g solute/100 g solvent or molarity. Affected by temperature (mostly increases for solids; decreases for gases), pressure (gases only — Henry's law), pH (for some). Saturation: maximum dissolved at given conditions; supersaturation: unstable excess. Foundation of many lab and industrial processes.
- Like dissolves likePolar dissolves polar; nonpolar dissolves nonpolar
- Maximum dissolvedSaturation point
- TemperatureSolid solubility usually increases with T
- Henry's lawGas solubility ∝ partial pressure
- Solvent factorsPolarity, dielectric, H-bonding capability
- ExamplesNaCl in water (high); oil in water (low)
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Why solubility matters
- Drug formulation. Bioavailability.
- Geology. Mineral dissolution, weathering.
- Environmental. Pollutant transport.
- Chemical engineering. Crystallization, separation.
- Biology. Aqueous transport in organisms.
- Industrial. Reaction engineering, washing.
- Cooking. Salt, sugar dissolution.
Common misconceptions
- Insoluble means zero. Always some dissolution; very low ≈ insoluble.
- Higher T always more dissolved. Gases decrease.
- Solvent unchanged when dissolving. Energetically affected.
- "Like dissolves like" absolute. Many exceptions; subtle factors.
- Saturation easy to determine. Slow equilibrium can mislead.
- Solvent doesn't affect rate. Stirring, T affect dissolution speed.
Frequently asked questions
What determines solubility?
Match between solvent and solute IMFs. (1) Solute-solvent attraction comparable to: solute-solute and solvent-solvent. (2) "Like dissolves like" rule. Polar solvents (water): dissolve polar (ethanol, glucose) and ionic (NaCl). Nonpolar solvents (hexane): dissolve nonpolar (oils, fats). Mixed: somewhere in between (e.g., 1-butanol moderate solubility in water).
How does temperature affect solubility?
Generally: solid solubility increases with T (most cases). Reason: increased thermal energy overcomes lattice energy. Exceptions: Na₂SO₄ (decreases above 32°C). Gas solubility decreases with T (gas molecules escape solution as T rises). Industrial: hot solutions can dissolve more solid; cold for keeping gases in.
What's Henry's law?
Gas solubility ∝ partial pressure of gas above liquid. C = kH × P. Higher partial P → more gas dissolved. Critical for: carbonated drinks (high CO₂ pressure → more dissolved CO₂; release pressure → fizz), scuba diving (pressure changes affect N₂ in blood, decompression sickness), respiration (O₂ in blood depends on partial pressure).
What's a saturated solution?
Maximum solute dissolved at given T and P. Adding more: doesn't dissolve; remains as solid (or out of solution). Below saturation: unsaturated (more can dissolve). Above: supersaturated (unstable; will eventually crystallize out). Equilibrium: solid ⇌ dissolved, where rate of dissolution = rate of precipitation.
How does pH affect solubility?
For salts of weak acids/bases. Adding acid: protonates -CO₂⁻ etc.; ionic compound becomes less ionic; less soluble. Example: salts of weak acids (like calcium carbonate) more soluble at low pH (acidic). Drug formulation: acidic vs basic environment affects bioavailability.
What's solubility product Ksp?
For sparingly soluble salts. AB(s) ⇌ A⁺(aq) + B⁻(aq). Ksp = [A⁺][B⁻]. Small Ksp = barely soluble. Large = soluble. Examples: CaCO₃ (Ksp 4.5×10⁻⁹ — barely soluble), CaSO₄ (Ksp 4.9×10⁻⁵ — moderately). Common ion effect: adding common ion shifts equilibrium back; reduces solubility.
How are solubilities measured?
(1) Saturate solution, filter, evaporate, weigh. (2) Spectroscopy of saturated solution. (3) Conductivity (for ionic solutes). (4) Phase diagrams. Tabulated values: g solute/100 g solvent or g/L or molarity. CRC Handbook has extensive tables for thousands of compounds.