Preparing Insoluble Salts
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge AS Level Physics 9702
What are Insoluble Salts? Insoluble salts are salts that do not dissolve in water. They form as solid precipitates when two soluble salt solutions are mixed. Unlike soluble salts, insoluble salts cannot be prepared using the methods of acid + metal, acid + base, or acid + alkali, because these methods rely on the salt dissolving in water.
Key Difference Soluble salts: Made by neutralization, then crystallization Insoluble salts: Made by precipitation reaction, no crystallization needed Precipitation Reactions A precipitation reaction occurs when two soluble salt solutions are mixed and an insoluble salt forms as a solid precipitate.
General Method Soluble Salt 1 (aq) + Soluble Salt 2 (aq) → Insoluble Salt (s) + Soluble Salt (aq) The key is choosing two soluble compounds that will react to form the desired insoluble salt. The Process Mix two soluble salt solutions that contain the ions needed The insoluble salt forms immediately as a precipitate Filter to separate the precipitate from the solution Wash with distilled water to remove soluble impurities Dry the precipitate in a warm oven or between filter paper No Crystallization Needed!
Unlike soluble salts, insoluble salts don't need crystallization because they immediately form as solid precipitates. You just filter, wash, and dry. Step-by-Step: Preparing Insoluble Salts Detailed Procedure Choose two soluble reactants One must contain the desired metal ion (cation) The other must contain the desired non-metal ion (anion) Mix the solutions Pour the two solutions together in a beaker Stir to ensure complete mixing The precipitate forms immediately Filter the mixture Use filter paper and a funnel The precipitate collects on the filter paper The solution (filtrate) passes through Wash the precipitate Rinse with distilled water several times This removes any soluble salts stuck to the precipitate Dry the precipitate Pat dry with filter paper, or Place in a warm oven (50-80°C) Leave until completely dry Example 1: Preparing Lead(II) Iodide Lead(II) iodide (PbI₂) is an insoluble yellow salt.
Planning We need Pb²⁺ ions → use soluble lead nitrate, Pb(NO₃)₂ We need I⁻ ions → use soluble potassium iodide, KI Equation Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq) lead nitrate + potassium iodide → lead iodide + potassium nitrate Ionic Equation Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s) Observations Yellow precipitate forms immediately The precipitate settles at the bottom The solution remains clear above the precipitate Procedure Mix lead nitrate solution with potassium iodide solution Filter to collect the yellow precipitate Wash with distilled water Dry in a warm oven Example 2: Preparing Silver Chloride Silver chloride (AgCl) is an insoluble white salt that turns purple in light.
Planning We need Ag⁺ ions → use soluble silver nitrate, AgNO₃ We need Cl⁻ ions → use soluble sodium chloride, NaCl (or any chloride) Equation AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) silver nitrate + sodium chloride → silver chloride + sodium nitrate Ionic Equation Ag⁺(aq) + Cl⁻(aq) → AgCl(s) Observations White precipitate forms immediately Precipitate is curdy in appearance The precipitate turns purple/grey in sunlight (photosensitive) Test for Chloride Ions This reaction is used to test for chloride ions .
Add silver nitrate solution - a white precipitate of silver chloride confirms Cl⁻ ions are present. Example 3: Preparing Barium Sulfate Barium sulfate (BaSO₄) is an insoluble white salt. Planning We need Ba²⁺ ions → use soluble barium chloride, BaCl₂ or barium nitrate, Ba(NO₃)₂ We need SO₄²⁻ ions → use any soluble sulfate (e.g., Na₂SO₄, MgSO₄, H₂SO₄) Equation BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq) barium chloride + sodium sulfate → barium sulfate + sodium chloride Alternative Equation BaCl₂(aq) + H₂SO₄(aq) → BaSO₄(s) + 2HCl(aq) barium chloride + sulfuric acid → barium sulfate + hydrochloric acid Ionic Equation Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) Observations White precipitate forms immediately Very fine, dense precipitate Test for Sulfate Ions This reaction is used to test for sulfate ions .
Add barium chloride solution (and a few drops of HCl) - a white precipitate of barium sulfate confirms SO₄²⁻ ions are present. Example 4: Preparing Calcium Carbonate Calcium carbonate (CaCO₃) is an insoluble white salt (chalk/limestone).
Planning We need Ca²⁺ ions → use soluble calcium chloride, CaCl₂ or calcium nitrate We need CO₃²⁻ ions → use soluble sodium carbonate, Na₂CO₃ Equation CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaCl(aq) calcium chloride + sodium carbonate → calcium carbonate + sodium chloride Ionic Equation Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s) Observations White precipitate forms immediately Fizzes if acid is added (CO₂ produced) Common Insoluble Salts Insoluble Salt Formula Color Soluble Reactants Needed Silver chloride AgCl White AgNO₃ + any chloride Silver bromide AgBr Cream AgNO₃ + any bromide Silver iodide AgI Yellow AgNO₃ + any iodide Lead(II) chloride PbCl₂ White Pb(NO₃)₂ + any chloride Lead(II…
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