We take some water in a petri dish and add some lead nitrate and potassium iodide to opposite ends of the dish as shown. After a while, we see that a yellow line appears in the centre of the dish, neatly dividing the dish into two parts. This is the precipitate of lead iodide that is formed due to the reaction of the two salts. The salts dissolve in water and the ions are diffused in the solution. As the ions move away from areas of high concentration to areas of low concentration, they meet each other and react to produce the yellow lead iodide precipitate in the centre of the dish.