When lead(II) nitrate solution is added to potassium iodide solid, a double displacement reaction occurs, forming lead(II) iodide and potassium nitrate. Lead(II) iodide is a bright yellow solid that is insoluble in water and remains as a precipitate in the reaction mixture. The overall reaction can be represented as follows:

Pb(NO3)2(aq) + 2KI(s) → PbI2(s) + 2KNO3(aq)

The products of the reaction, lead(II) iodide and potassium nitrate, are both soluble in water. However, due to the low solubility of lead(II) iodide, it precipitates out of the solution and remains as a solid. The potassium nitrate remains dissolved in the water, forming a colourless solution. This reaction is commonly used to test for the presence of lead ions in a solution, as the formation of the bright yellow precipitate of lead(II) iodide is a clear indication of the presence of lead.