Solubility Rules

Solubility tells us if a substance dissolves in water. These rules help you predict if a salt will dissolve, form crystals, or make a solid (a precipitate) when two solutions are mixed. Think of salts as passengers: some always get on the “water bus”, some never do, and a few are fussy.

Key Ideas

• Soluble: dissolves in water. Insoluble: does not dissolve (or only a tiny amount).
• Use these rules to choose preparation methods (crystallisation for soluble salts; precipitation for insoluble salts) and to predict precipitates.

Core Solubility Rules

• All sodium (Na⁺), potassium (K⁺) and ammonium (NH₄⁺) salts are soluble.
• All nitrates (NO₃⁻) are soluble.
• Chlorides (Cl⁻) are soluble, except silver and lead chlorides (insoluble).
• Sulfates (SO₄²⁻) are soluble, except barium, calcium and lead sulfates (insoluble).
• Carbonates (CO₃²⁻) are insoluble, except those of sodium, potassium, ammonium.
• Hydroxides (OH⁻) are insoluble, except sodium, potassium, ammonium and calcium (calcium hydroxide is only slightly soluble).

Quick Applications

• Is AgCl soluble? Chloride with silver ⇒ insoluble.
• Is K₂SO₄ soluble? Potassium salt ⇒ soluble.
• Is BaSO₄ soluble? Sulfate with barium ⇒ insoluble.
• Is Na₂CO₃ soluble? Sodium carbonate ⇒ soluble.
• Is Ca(OH)₂ soluble? Only slightly; gives an alkaline solution but much remains undissolved.

Common Misconceptions

• “Insoluble” means almost none dissolves, not zero.
• Solubility is not the same as speed of dissolving.
• Exceptions matter: do not assume all chlorides or all sulfates are soluble.