It's raining gold!

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Safe­ty pre­cau­tions
Wear pro­tec­tive glass­es and gloves. Lead com­pounds are very poi­sonous! Don’t swal­low the salts and so­lu­tions! Fol­low gen­er­al safe­ty rec­om­men­da­tions. Chem­i­cal ex­per­i­ments must be car­ried out in full com­pli­ance with the leg­is­la­tion of your coun­try.

Warn­ing! Sub­stances of this ex­per­i­ment are tox­ic and high­ly dan­ger­ous for your health. Do not try this at home. Only un­der pro­fes­sion­al su­per­vi­sion.

When a hot so­lu­tion of lead io­dide cools down, the salt set­tles in beau­ti­ful gold­en par­ti­cles.

Reagents and equip­ment:
* lead ni­trate (0.3 g );
* potas­si­um io­dide (0.3 g);
* acetic acid (~1-2 ml);
* dis­tilled wa­ter (500 ml);
* two beakers;
* flask (500 ml).

Step-by-step in­struc­tions
In dif­fer­ent glass­es, dis­solve lead ni­trate and potas­si­um io­dide in small quan­ti­ties of wa­ter. Add a lit­tle acetic acid to the so­lu­tion, to pre­vent its hy­drol­y­sis. Heat the re­main­ing dis­tilled wa­ter to ~80°С. Mix the so­lu­tions of lead ni­trate and potas­si­um io­dide in a large flask. A sus­pen­sion of a yel­low sed­i­ment forms. Add hot wa­ter un­til the sed­i­ment dis­solves com­plete­ly. Let the so­lu­tion cool. When it cools, beau­ti­ful gold crys­tals of lead io­dide be­gin to set­tle.

Pro­cess­es de­scrip­tion
When the so­lu­tions of the two salts are mixed, an ex­change re­ac­tion takes place:

Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃

The form­ing lead io­dide is in­sol­u­ble in cold wa­ter, but at a high tem­per­a­ture its sol­u­bil­i­ty in­creas­es. So in a hot so­lu­tion, pre­cip­i­ta­tion is not ob­served. When the mix­ture cools, the sol­u­bil­i­ty of lead io­dide de­creas­es, and Pbl₂ crys­tals grad­u­al­ly be­gin to set­tle.