Precipitation Reactions — Forming Insoluble Products
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Precipitation reactions occur when two soluble ionic compounds react in solution to form an insoluble solid product called a precipitate. The precipitate forms because the product's lattice energy exceeds its hydration energy — the ions prefer to bond with each other rather than remain surrounded by water molecules. Solubility rules, derived from extensive experimental data, predict which combinations produce precipitates.
Reaction Mechanism
When two solutions are mixed, all four possible ion combinations exist momentarily in solution. If one combination has low solubility, those ions rapidly nucleate and grow into solid crystals that settle out. The rate of precipitation depends on supersaturation, temperature, and mixing. Adding reagent slowly produces larger, purer crystals (useful in gravimetric analysis); rapid mixing produces fine precipitates. The solubility product constant (Ksp) quantifies the equilibrium between dissolved ions and solid.
Everyday Examples
Hard water stains on glass are calcium carbonate precipitated when dissolved calcium bicarbonate loses CO2. Kidney stones are precipitates of calcium oxalate or calcium phosphate that form when urine becomes supersaturated. Mixing silver nitrate with table salt (NaCl) instantly produces a white curdy precipitate of silver chloride — a reaction used in photography since the 1840s.
工业重要性
Wastewater treatment relies heavily on precipitation to remove phosphates, heavy metals, and fluoride. Adding lime to industrial effluent precipitates toxic metal hydroxides for safe disposal. Gravimetric analysis — precipitating, filtering, and weighing a compound — remains the gold standard for accurate quantitative determination of many analytes in regulatory testing.
Safety Note
Some precipitates contain toxic heavy metals (lead chromate, barium sulfate from barium chloride). Handle precipitates with gloves and dispose of them as chemical waste. Avoid inhaling fine precipitate dust.
Iron(III) Hydroxide Precipitation
FeCl₃ + 3NaOH → Fe(OH)₃↓ + 3NaCl
Iron(III) chloride reacts with sodium hydroxide to form a rust-brown gelatinous precipitate of iron(III) hydroxide. Fe(OH)₃ has an extremely low …
Aluminum Hydroxide Precipitation
AlCl₃ + 3NaOH → Al(OH)₃↓ + 3NaCl
Aluminum chloride reacts with sodium hydroxide to form a white gelatinous precipitate of aluminum hydroxide (Ksp = 3 × 10⁻³⁴). …
Barium Carbonate Precipitation
BaCl₂ + Na₂CO₃ → BaCO₃↓ + 2NaCl
Barium chloride reacts with sodium carbonate to form a white precipitate of barium carbonate (Ksp = 2.6 × 10⁻⁹). BaCO₃ …
Barium Sulfate Precipitation
BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl
Barium chloride reacts with sodium sulfate to form an extremely insoluble white precipitate of barium sulfate (Ksp = 1.1 × …
Cadmium Sulfide Precipitation (Yellow)
CdCl₂ + Na₂S → CdS↓ + 2NaCl
Cadmium chloride reacts with sodium sulfide to form a bright yellow precipitate of cadmium sulfide (Ksp = 8 × 10⁻²⁷). …
Calcium Fluoride Precipitation (Fluorite)
CaCl₂ + 2NaF → CaF₂↓ + 2NaCl
Calcium chloride reacts with sodium fluoride to precipitate calcium fluoride (Ksp = 3.5 × 10⁻¹¹). CaF₂ occurs naturally as the …
Calcium Phosphate Precipitation (Bone Mineral)
3CaCl₂ + 2Na₃PO₄ → Ca₃(PO₄)₂↓ + 6NaCl
Calcium chloride reacts with trisodium phosphate to form a white precipitate of calcium phosphate (Ksp = 2.1 × 10⁻³³). Calcium …
Chromium(III) Hydroxide Precipitation
CrCl₃ + 3NaOH → Cr(OH)₃↓ + 3NaCl
Chromium(III) chloride reacts with sodium hydroxide to form a green gelatinous precipitate of chromium(III) hydroxide (Ksp = 6.3 × 10⁻³¹). …
Lead Carbonate Precipitation
Pb(NO₃)₂ + Na₂CO₃ → PbCO₃↓ + 2NaNO₃
Lead nitrate reacts with sodium carbonate to form a white precipitate of lead carbonate (Ksp = 7.4 × 10⁻¹⁴). Basic …
Lead Chloride Precipitation
Pb(NO₃)₂ + 2NaCl → PbCl₂↓ + 2NaNO₃
Lead nitrate reacts with sodium chloride to form a white precipitate of lead chloride (Ksp = 1.7 × 10⁻⁵). PbCl₂ …
Calcium Carbonate Precipitation
CaCl₂ + Na₂CO₃ → CaCO₃↓ + 2NaCl
Calcium chloride reacts with sodium carbonate to precipitate calcium carbonate. CaCO₃ has a Ksp of 3.4 × 10⁻⁹. This is …
Silver Bromide Precipitation
AgNO₃ + NaBr → AgBr↓ + NaNO₃
Silver bromide precipitates as a pale yellow solid with Ksp = 5.4 × 10⁻¹³. It is much less soluble than …
Magnesium Hydroxide Precipitation
MgCl₂ + 2NaOH → Mg(OH)₂↓ + 2NaCl
Magnesium chloride reacts with sodium hydroxide to form a white precipitate of magnesium hydroxide (Ksp = 5.6 × 10⁻¹²). The …
Zinc Hydroxide Precipitation
ZnSO₄ + 2NaOH → Zn(OH)₂↓ + Na₂SO₄
Zinc sulfate reacts with sodium hydroxide to form a white gelatinous precipitate of zinc hydroxide (Ksp = 3.0 × 10⁻¹⁷). …
Calcium Sulfate Precipitation (Gypsum)
Ca(NO₃)₂ + Na₂SO₄ → CaSO₄↓ + 2NaNO₃
Calcium nitrate reacts with sodium sulfate to precipitate calcium sulfate (Ksp = 4.9 × 10⁻⁵). CaSO₄ is sparingly soluble, not …
Copper Sulfide Precipitation
CuSO₄ + Na₂S → CuS↓ + Na₂SO₄
Copper sulfate reacts with sodium sulfide to form a black precipitate of copper(II) sulfide (Ksp = 1.3 × 10⁻³⁶). CuS …
Strontium Sulfate Precipitation
SrCl₂ + Na₂SO₄ → SrSO₄↓ + 2NaCl
Strontium chloride reacts with sodium sulfate to form a white precipitate of strontium sulfate (Ksp = 3.4 × 10⁻⁷). SrSO₄ …
Cobalt(II) Hydroxide Precipitation
CoCl₂ + 2NaOH → Co(OH)₂↓ + 2NaCl
Cobalt(II) chloride reacts with NaOH to form a blue or pink precipitate of cobalt(II) hydroxide (Ksp = 5.9 × 10⁻¹⁶). …
Silver Iodide Precipitation
AgNO₃ + KI → AgI↓ + KNO₃
Silver nitrate reacts with potassium iodide to form a yellow precipitate of silver iodide (Ksp = 8.5 × 10⁻¹⁷). AgI …
Silver Chromate Precipitation
2AgNO₃ + K₂CrO₄ → Ag₂CrO₄↓ + 2KNO₃
Silver nitrate reacts with potassium chromate to form a brick-red precipitate of silver chromate (Ksp = 1.1 × 10⁻¹²). This …
Mercury(II) Sulfide Precipitation (Vermilion)
HgCl₂ + Na₂S → HgS↓ + 2NaCl
Mercury(II) chloride reacts with sodium sulfide to form a black precipitate of mercury(II) sulfide (Ksp = 2 × 10⁻⁵²), the …
Lead Sulfate Precipitation
Pb(NO₃)₂ + Na₂SO₄ → PbSO₄↓ + 2NaNO₃
Lead nitrate reacts with sodium sulfate to form a white precipitate of lead sulfate (Ksp = 2.5 × 10⁻⁸). PbSO₄ …
Calcium Oxalate Precipitation (Kidney Stones)
CaCl₂ + Na₂C₂O₄ → CaC₂O₄↓ + 2NaCl
Calcium chloride reacts with sodium oxalate to form a white precipitate of calcium oxalate (Ksp = 2.3 × 10⁻⁹). Calcium …
Ferric Phosphate Precipitation (Water Treatment)
FeCl₃ + Na₃PO₄ → FePO₄↓ + 3NaCl
Iron(III) chloride reacts with trisodium phosphate to form an insoluble yellowish-white precipitate of iron(III) phosphate (Ksp = 9.9 × 10⁻¹⁶). …
Lead Iodide Precipitation (Golden Rain)
Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃
Lead nitrate and potassium iodide form a bright yellow precipitate of lead iodide. When the hot mixture is cooled slowly, …
Copper(II) Hydroxide Precipitation
CuSO₄ + 2NaOH → Cu(OH)₂↓ + Na₂SO₄
Copper sulfate reacts with sodium hydroxide to form a pale blue gelatinous precipitate of copper(II) hydroxide (Ksp = 2.2 × …
Nickel Hydroxide Precipitation
NiCl₂ + 2NaOH → Ni(OH)₂↓ + 2NaCl
Nickel chloride reacts with sodium hydroxide to form a green precipitate of nickel(II) hydroxide (Ksp = 5.5 × 10⁻¹⁶). The …
Silver Chloride Precipitation
AgNO₃ + NaCl → AgCl↓ + NaNO₃
Silver nitrate reacts with sodium chloride to form a white curdy precipitate of silver chloride. AgCl has a very low …
Manganese Dioxide Precipitation
MnSO₄ + 2NaOH + H₂O₂ → MnO₂↓ + Na₂SO₄ + 2H₂O
Manganese(II) sulfate is oxidized by hydrogen peroxide in basic solution to form a dark brown precipitate of manganese dioxide. Mn …
Zinc Sulfide Precipitation
ZnSO₄ + Na₂S → ZnS↓ + Na₂SO₄
Zinc sulfate reacts with sodium sulfide to form a white precipitate of zinc sulfide (Ksp = 2 × 10⁻²⁵). ZnS …
Iron(II) Sulfide Precipitation
FeSO₄ + Na₂S → FeS↓ + Na₂SO₄
Iron(II) sulfate reacts with sodium sulfide to form a black precipitate of iron(II) sulfide (Ksp = 6 × 10⁻¹⁸). FeS …