MODEL RELEASED. Potassium displacing lead. Student adding potassium iodide solution (KI) to a test tube of lead (II) nitrate solution (Pb[NO3]2). The yellow precipitate of lead (II) iodide (PbI2) is formed because the potassium is more reactive than the lead and also because the formation of a solid drives the reaction in that direction. A solution of potassium nitrate (KNO3) forms in what is known as a double displacement (substitution) reaction. The equation for this reaction is: 2 KI + Pb(NO3)2 ----> 2 KNO3 + PbI2 Such experiments allow metals to be ranked within a reactivity series.