Solubility Rules
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge AS Level Physics 9702
What is Solubility? Solubility is the ability of a substance to dissolve in a solvent (usually water) to form a solution. A substance is described as: Soluble - if it dissolves readily in water Insoluble - if it does not dissolve in water (or only very slightly) Sparingly soluble - if only a small amount dissolves Important!
"Insoluble" doesn't mean zero solubility - it means the solubility is so low that we can consider the substance as not dissolving. Some substances described as "insoluble" do dissolve very slightly. Why Learn Solubility Rules?
Solubility rules help us: Predict which salts will dissolve in water and which will form precipitates Choose the correct method for preparing salts Predict the products of precipitation reactions Identify ions through chemical tests Key Application To prepare an insoluble salt by precipitation, you need to know which salts are soluble (to use as reactants) and which are insoluble (the product you want).
General Solubility Rules Rule 1: All Nitrates are Soluble All nitrates (NO₃⁻) are soluble in water Examples: NaNO₃, KNO₃, AgNO₃, Pb(NO₃)₂, Ca(NO₃)₂ This is why nitrates are commonly used in precipitation reactions - they provide a soluble source of metal ions.
Rule 2: All Sodium, Potassium, and Ammonium Salts are Soluble All salts of Na⁺, K⁺, and NH₄⁺ are soluble Examples: NaCl, Na₂SO₄, KBr, K₂CO₃, (NH₄)₂SO₄, NH₄Cl These are Group 1 metals plus ammonium. Their salts always dissolve regardless of the anion.
Rule 3: Most Chlorides, Bromides, and Iodides are Soluble Most halides (Cl⁻, Br⁻, I⁻) are soluble Exceptions: Silver and lead(II) halides are insoluble Insoluble: AgCl, AgBr, AgI, PbCl₂, PbBr₂, PbI₂ Rule 4: Most Sulfates are Soluble Most sulfates (SO₄²⁻) are soluble Exceptions: Barium, calcium, and lead(II) sulfates are insoluble Insoluble: BaSO₄, CaSO₄, PbSO₄ Rule 5: Most Carbonates are Insoluble Most carbonates (CO₃²⁻) are insoluble Exceptions: Sodium, potassium, and ammonium carbonates are soluble Soluble: Na₂CO₃, K₂CO₃, (NH₄)₂CO₃ Insoluble: CaCO₃, MgCO₃, CuCO₃, ZnCO₃, FeCO₃ Rule 6: Most Hydroxides are Insoluble Most hydroxides (OH⁻) are insoluble Exceptions: Sodium, potassium, and calcium hydroxides are soluble Soluble: NaOH, KOH, Ca(OH)₂ Insoluble: Cu(OH)₂, Fe(OH)₂, Fe(OH)₃, Zn(OH)₂, Al(OH)₃ Calcium Hydroxide Calcium hydroxide Ca(OH)₂ is slightly soluble .
It forms a weakly alkaline solution called limewater. Summary Table: Solubility Rules Compound Type General Solubility Exceptions Nitrates (NO₃⁻) All soluble None Sodium salts (Na⁺) All soluble None Potassium salts (K⁺) All soluble None Ammonium salts (NH₄⁺) All soluble None Chlorides (Cl⁻) Most soluble AgCl, PbCl₂ (insoluble) Bromides (Br⁻) Most soluble AgBr, PbBr₂ (insoluble) Iodides (I⁻) Most soluble AgI, PbI₂ (insoluble) Sulfates (SO₄²⁻) Most soluble BaSO₄, CaSO₄, PbSO₄ (insoluble) Carbonates (CO₃²⁻) Most insoluble Na₂CO₃, K₂CO₃, (NH₄)₂CO₃ (soluble) Hydroxides (OH⁻) Most insoluble NaOH, KOH, Ca(OH)₂ (soluble) Detailed Solubility Information Chlorides, Bromides, and Iodides (Halides) Salt Chloride (Cl⁻) Bromide (Br⁻) Iodide (I⁻) Sodium NaCl - Soluble NaBr - Soluble NaI - Soluble Potassium KCl - Soluble KBr - Soluble KI - Soluble Silver AgCl - Insoluble AgBr - Insoluble AgI - Insoluble Lead(II) PbCl₂ - Insoluble PbBr₂ - Insoluble PbI₂ - Insoluble Calcium CaCl₂ - Soluble CaBr₂ - Soluble CaI₂ - Soluble Magnesium MgCl₂ - Soluble MgBr₂ - Soluble MgI₂ - Soluble Remember Only silver and lead(II) halides are insoluble.
All other halides dissolve. Sulfates Metal Sulfate Formula Solubility Sodium Na₂SO₄ Soluble Potassium K₂SO₄ Soluble Ammonium (NH₄)₂SO₄ Soluble Magnesium MgSO₄ Soluble Copper(II) CuSO₄ Soluble Zinc ZnSO₄ Soluble Iron(II) FeSO₄ Soluble Barium BaSO₄ Insoluble Calcium CaSO₄ Insoluble Lead(II) PbSO₄ Insoluble Carbonates Metal Carbonate Formula Solubility Sodium Na₂CO₃ Soluble Potassium K₂CO₃ Soluble Ammonium (NH₄)₂CO₃ Soluble Calcium CaCO₃ Insoluble Magnesium MgCO₃ Insoluble Copper(II) CuCO₃ Insoluble Zinc ZnCO₃ Insoluble Iron(II) FeCO₃ Insoluble Lead(II) PbCO₃ Insoluble Using Solubility Rules Example 1: Predicting Solubility Q: Is copper(II) sulfate soluble or insoluble?
A: It's a sulfate (SO₄²⁻) Most sulfates are soluble The exceptions are Ba, Ca, and Pb sulfates Copper is NOT in the exception list Therefore, CuSO₄ is soluble Example 2: Predicting a Precipitate Q: What happens when silver nitrate solution is mixed with sodium chloride solution?
A: AgNO₃(aq) + NaCl(aq) → AgCl + NaNO₃ Check if AgCl is soluble: It's a chloride, and AgCl is an exception → insoluble Check if NaNO₃ is soluble: It's a sodium salt and a nitrate → soluble AgCl will form a white precipitate NaNO₃ remains in solution Example 3: Choosing Reactants Q: What two soluble compounds could you use to prepare barium sulfate?
A: Need Ba²⁺ ions from a soluble barium compound BaSO₄ is insoluble, but barium nitrate and barium chloride are soluble Choose: BaCl₂ or Ba(NO₃)₂ Need SO₄²⁻ ions from a soluble sulfate Most sulfates are soluble …
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