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.

Tầm quan trọng công nghiệp

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 …

Tỏa nhiệt · ΔH = -88,0 kJ

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). …

Tỏa nhiệt · ΔH = -115,0 kJ

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 …

Tỏa nhiệt · ΔH = -17,0 kJ

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 × …

Tỏa nhiệt · ΔH = -24,5 kJ

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). …

Tỏa nhiệt · ΔH = -78,0 kJ

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 …

Tỏa nhiệt · ΔH = -18,0 kJ

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 …

Tỏa nhiệt · ΔH = -105,0 kJ

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). …

Tỏa nhiệt · ΔH = -90,0 kJ

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 …

Tỏa nhiệt · ΔH = -22,0 kJ

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 …

Tỏa nhiệt · ΔH = -14,0 kJ

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 …

Tỏa nhiệt · ΔH = -13,0 kJ

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 …

Tỏa nhiệt · ΔH = -85,0 kJ

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 …

Tỏa nhiệt · ΔH = -41,0 kJ

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). …

Tỏa nhiệt · ΔH = -38,0 kJ

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 …

Tỏa nhiệt · ΔH = -8,0 kJ

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 …

Tỏa nhiệt · ΔH = -84,0 kJ

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 …

Tỏa nhiệt · ΔH = -9,5 kJ

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). …

Tỏa nhiệt · ΔH = -52,0 kJ

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 …

Tỏa nhiệt · ΔH = -110,0 kJ

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 …

Tỏa nhiệt · ΔH = -35,0 kJ

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 …

Tỏa nhiệt · ΔH = -83,0 kJ

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 …

Tỏa nhiệt · ΔH = -19,0 kJ

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 …

Tỏa nhiệt · ΔH = -12,0 kJ

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). …

Tỏa nhiệt · ΔH = -95,0 kJ

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, …

Tỏa nhiệt · ΔH = -32,0 kJ

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 × …

Tỏa nhiệt · ΔH = -59,0 kJ

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 …

Tỏa nhiệt · ΔH = -55,0 kJ

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 …

Tỏa nhiệt · ΔH = -65,5 kJ

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 …

Tỏa nhiệt · ΔH = -200,0 kJ

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 …

Tỏa nhiệt · ΔH = -65,0 kJ

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 …

Tỏa nhiệt · ΔH = -75,0 kJ