Solubility

Core Concept

Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a specified temperature, producing a saturated solution. Solubility depends on the interplay of solute–solute, solvent–solvent, and solute–solvent forces ("like dissolves like"), and it responds to temperature and pressure in ways that differ sharply between gases and solids: gas solubility decreases with temperature and increases with pressure, while most (but not all) solids become more soluble as temperature rises.

Key Ideas

Saturation terminology

• Unsaturated: less than the maximum solute; more can dissolve.
• Saturated: the maximum solute is dissolved; undissolved solute and solution are in dynamic equilibrium.
• Supersaturated: more than the equilibrium amount is dissolved (unstable; seeding causes sudden crystallization).

Temperature dependence

• Solids: solubility usually increases with temperature (KNO₃, sugar), but not always — some salts (e.g., Ce₂(SO₄)₃, some CaSO₄) become less soluble when heated.
• Gases: solubility always decreases with increasing temperature (warm soda goes flat; warm water holds less O₂).

Pressure dependence

• Gases: solubility increases with the partial pressure of the gas (Henry's law, C = k_H·P).
• Solids/liquids: pressure has negligible effect (they are nearly incompressible).

Solubility rules (ionic compounds in water)

• Most alkali-metal (Li⁺, Na⁺, K⁺…) and ammonium (NH₄⁺) salts and most nitrates (NO₃⁻) are soluble.
• Most chlorides are soluble (exceptions: AgCl, PbCl₂, Hg₂Cl₂).
• Most sulfates are soluble (exceptions: BaSO₄, PbSO₄, CaSO₄ slightly).
• Most carbonates, phosphates, hydroxides, and sulfides are insoluble (except with alkali metals/NH₄⁺).

Equations and Variables

• Solubility (s): grams solute per 100 g solvent (common units) or mol/L (molar solubility).
• Henry's law (gas solubility vs. pressure): C = k_H·P, where C = dissolved-gas concentration, k_H = Henry's law constant (temperature-dependent), P = partial pressure of the gas.
• Supersaturation has no equation; it is a kinetic metastable state reached by cooling a saturated solution or evaporating solvent slowly.

How It Works

1. A solute dissolves until equilibrium between dissolved solute and undissolved solid is reached — the saturation limit.
2. Heating usually lets more solid dissolve because the increased thermal motion helps overcome lattice forces; but if the dissolution is exothermic, heating instead decreases solubility (Le Chatelier).
3. Heating always drives gases out because dissolved gas molecules gain enough energy to escape the solvent.
4. Pressurizing a gas above a solution forces more gas in (Henry's law) — more collisions, more dissolved gas.
5. Cooling or concentrating a saturated solution can produce supersaturation, which crystallizes abruptly when disturbed.

Worked Example

A solution is made with 80 g of KNO₃ in 100 g of water at 60 °C and then cooled to 20 °C. The solubility of KNO₃ is 110 g/100 g H₂O at 60 °C and 32 g/100 g H₂O at 20 °C. How much KNO₃ crystallizes?

1. At 60 °C, 80 g dissolved in 100 g water is below the 110 g limit → the solution is unsaturated (no solid present).
2. Cool to 20 °C, where only 32 g can stay dissolved.
3. Amount that must precipitate = 80 g − 32 g = 48 g of KNO₃ crystallizes out.

How it works

1. A solute dissolves until equilibrium between dissolved solute and undissolved solid is reached — the saturation limit.
2. Heating usually lets more solid dissolve because the increased thermal motion helps overcome lattice forces; but if the dissolution is exothermic, heating instead decreases solubility (Le Chatelier).
3. Heating always drives gases out because dissolved gas molecules gain enough energy to escape the solvent.
4. Pressurizing a gas above a solution forces more gas in (Henry's law) — more collisions, more dissolved gas.
5. Cooling or concentrating a saturated solution can produce supersaturation, which crystallizes abruptly when disturbed.

Common confusions

• "Heating always increases solubility." — Wrong. Gases always become less soluble when heated, and even some solids (whose dissolution is exothermic) decrease.
• "Pressure strongly changes solid solubility." — Wrong. Pressure matters for gases (Henry's law); solids and liquids are nearly unaffected.
• "A saturated solution has no dissolved solute." — Wrong. Saturated means the maximum is dissolved; the solute is fully present, just at its limit.
• "All chlorides are soluble." — Wrong. AgCl, PbCl₂, and Hg₂Cl₂ are notable insoluble exceptions.
• "Supersaturated solutions are the same as saturated." — Wrong. Supersaturated holds more than the equilibrium amount and is metastable — it will crystallize if disturbed.

Quick review

• Saturated = max dissolved; unsaturated < max; supersaturated > max (unstable).
• Gases: solubility ↓ T, ↑ P (Henry's law). Solids: usually ↑ T, P negligible.
• Solubility rules: alkali/NH₄⁺ and nitrates soluble; AgCl/PbCl₂/BaSO₄ and most carbonates/hydroxides insoluble.
• Worked: cooling 80 g KNO₃ (60→20 °C) precipitates 48 g.