Golden Rain
Summary
Lead(II) iodide is a vivid golden precipitate1 that dissolves in hot water and re-crystallizes as it cools, showering down as glittering flakes. You’ll be able to explain why its solubility2 climbs so steeply with temperature, what slow cooling does to crystal size, and why the solid is gold rather than white.
History
The golden rain reaction has been a classroom demonstration for well over a century, appearing in chemistry textbooks since the late 1800s. Lead(II) iodide’s vivid golden-yellow color — far more saturated than the pale yellow of silver iodide — made it a natural showpiece for precipitation chemistry.
The dramatic “rain” effect depends on an unusual solubility curve: PbI₂ is nearly insoluble in cold water (K_sp = 9.8 × 10⁻⁹ at 25°C) but becomes significantly more soluble as temperature rises. Dissolving it in near-boiling water then cooling slowly produces the slow nucleation and crystal growth that gives the reaction its name. The bright color comes from the electronic structure of the lead–iodide bond: the large, polarizable iodide ion shifts the charge-transfer absorption band well into the visible range, giving the solid its striking gold color.
Hazards & preparation
Lead compounds are toxic — this is a high-hazard experiment.
- PPE: gloves and safety goggles throughout; work on a covered surface.
- Do not ingest or inhale lead(II) nitrate or lead(II) iodide, and avoid prolonged skin contact. This one is best for older students with supervision.
Disposal: never pour lead-containing solutions down the drain. Let the cooled suspension settle or filter it, and hold the lead solid and liquid for heavy-metal hazardous-waste collection. The potassium nitrate filtrate is low-hazard but still keep it out of waterways. See the Safety page.
Materials
- Lead(II) nitrate — 0.5 g
- Potassium iodide — 0.5 g (or sodium iodide as a substitute)
- Distilled water — 200 mL
- 250 mL beaker or clear Erlenmeyer flask
- Hot plate or kettle
- Gloves, safety goggles
Procedure
- Heat 200 mL distilled water to near boiling in the beaker.
- Add the lead(II) nitrate and stir to dissolve — the solution should be clear.
- Add the potassium iodide — a vivid yellow PbI₂ precipitate forms immediately.
- Keep heating and stirring until the yellow precipitate fully dissolves; the solution turns faintly yellow and clear. (If it stays cloudy, add a little hot water and keep stirring.)
- Remove from heat and set on an undisturbed surface.
- As it cools, golden-yellow PbI₂ crystals nucleate and drift slowly down like a shower of gold.
Tip: cool slowly without stirring for the most dramatic effect — rapid cooling or agitation gives a fine powder instead of falling crystals.
What you should see
A brief flash of yellow precipitate when the iodide goes in, then a clear, faintly-yellow solution once it’s all dissolved hot. On cooling, sparkling gold platelets appear throughout the liquid and sink gently — a slow, glittering “rain” that continues for minutes.
| Symptom | Likely cause | Fix |
|---|---|---|
| Precipitate won’t redissolve when hot | Not hot enough, or too little water | Heat closer to boiling; add a little more hot water |
| Fine cloudy powder, no falling flakes | Cooled too fast or stirred | Cool slowly and undisturbed for large crystals |
| Solution stays clear on cooling | Too dilute | Use the full 0.5 g of each; less water |
The reactions
\[\ce{Pb(NO3)2(aq) + 2 KI(aq) -> PbI2(s) v + 2 KNO3(aq)}\]
On heating:
\[\ce{PbI2(s) ->[\Delta][H2O] Pb^{2+}(aq) + 2 I^{-}(aq)}\]
On cooling:
\[\ce{Pb^{2+}(aq) + 2 I^{-}(aq) -> PbI2(s) v}\] (golden crystals)
lead nitrate + potassium iodide → golden lead iodide + potassium nitrate; the lead iodide dissolves when hot and re-crystallizes when cool
The Science
PbI₂ has a strongly temperature-dependent solubility. At 25°C the solubility product is ~10 × 10⁻⁹, corresponding to only about 0.6 g per liter; by 100°C this rises more than tenfold. When the hot solution cools, the solution becomes supersaturated — Pb²⁺ and I⁻ ions are present at concentrations far above the room-temperature equilibrium. Nucleation is slow in a clean, undisturbed solution, so crystals form gradually at scattered sites and grow large enough to be visible as they sink. The shower continues until equilibrium is restored.
The vivid color arises from the same physics as in silver iodide and other lead(II) halides: the large, polarizable iodide ligand causes a ligand-to-metal charge-transfer absorption that reaches well into the visible spectrum, absorbing blue and violet light and reflecting the yellow–gold remainder.
Questions to Explore
Why does solubility increase with temperature here? PbI₂ becomes nearly ten-fold more soluble between 25°C and 100°C. What does hot water do that cold water doesn’t when dissolving an ionic solid?
Hint / answer
Dissolving PbI₂ absorbs heat (endothermic), so extra thermal energy helps break the ions out of the crystal and keep them in solution. Hotter water supplies that energy, so far more dissolves; cooling withdraws it and the salt drops back out.
What is a solubility product? With Ksp = [Pb²⁺][I⁻]², why does the exponent on iodide matter, and what happens if you double only the iodide?
Hint / answer
The squared term means iodide counts twice over — two iodides per lead. Doubling [I⁻] multiplies the product by four, so it pushes precipitation much harder than doubling lead would. Ksp sets the ceiling: exceed it and solid must form.
Why large crystals rather than a fine precipitate? Slow undisturbed cooling gives visible falling crystals; fast/stirred cooling gives powder. What decides crystal size?
Hint / answer
Slow cooling creates only a few nucleation sites, so each grows large before the supersaturation is used up. Fast cooling or stirring spawns countless tiny nuclei at once, so the salt is shared among many crystals and each stays small.
Why is PbI₂ golden when PbCl₂ and PbBr₂ are white? What’s special about iodide?
Hint / answer
Iodide is large and highly polarizable, so its electrons shift toward lead easily, lowering the energy of light the solid absorbs into the visible (blue) range — leaving it looking gold. Chloride and bromide are smaller and harder to polarize, so they only absorb ultraviolet and appear white.
What makes the “rain” fall slowly? Individual crystals drift down rather than dropping instantly. What forces act on a falling crystal?
Hint / answer
Gravity pulls each crystal down while the water’s buoyancy and drag resist it, so it settles at a slow terminal velocity. Smaller flakes have more drag relative to their weight, so they fall slower — giving the drifting, shimmering “rain.”
Going further
- Tune the crystal size. Cool one batch slowly on the bench and another in an ice bath, and compare flake size and how the “rain” looks.
- Next in the Crystals & Solutions track: grow dendrites and sublimation needles in Menthol Crystal Patterns — crystallization with no toxic metals.
Footnotes
Precipitate — An insoluble solid that forms and separates out when two solutions are mixed.↩︎
Solubility — The maximum amount of a substance that will dissolve in a given amount of solvent at a given temperature.↩︎