What is the yellow powder made from lead nitrate and potassium iodide used for?

Answer 1

You must be thinking of the classical precipitation reaction done in many freshmen chemistry science labs, the mixture of lead nitrate and potassium iodide:

* Pb(NO3)2(s) + 2 KI(s) → PbI2(s) + 2 KNO3(s) The precipitate, lead (II) iodide (PbI2) is a toxic, yellowish solid. Lead iodide is toxic due to its lead content. It displays a range of colors with varying temperature from bright yellow at room temperature to brick red. On cooling, its color returns to yellow. Currently, it only finds use in its crystalline form, where it is used as a detector material for high energy photons including x-rays and gamma rays. In the nineteenth century it was used as an artists' pigment under the name Iodine Yellow, but it was too unstable to be useful.