The reaction shown in the demonstration is lead to nitrate plus two potassium iodide gives lead iodide plus two potassium nitrate. Notice the two negative ions, NO3 minus and I- have exchanged places. Now, write the complete ionic equation and then the net ionic equation. So we have Pb2+ plus two NO3 minus plus two potassium+ plus two I minus. This gives us PbI2 plus two potassium+ plus two NO3-. We cancel the two NO3- and the two K+ on each side of the equation, and we come up with the net ionic equation, which showed the actual reaction that occurred in our solution. The colorless solution of Pb2+ plus the colorless solution of 2 I- gave the deep yellow color of PbI2 solid. This reaction is driven by the formation of the precipitate or the solid. Well, how could we predict this reaction without doing the experiment? Obviously we'd like to save some work if we can. Well, we can make predictions based on the solubility of compounds, which have been determined experimentally. These are the common solubility rules found in your text and laboratory manual. Let's illustrate how we can predict that lead (II) nitrate reacts with potassium iodide in aqueous solution to give the yellow precipitate lead (II) iodide, PbI2. First, write all the ions that would be in the solution together, just before any possible reaction occurs. These would be Pb2+, NO3-, potassium+ or K+ and I-. Second, ask yourself the question do any of the positive or negative ion combinations result in an insoluble compound? This is where the solubility rules come in. The rules tell us that lead (II) halides are generally insoluble and that all nitrates are soluble. This means that Pb2+ and I minus cannot exist in the solution together at reasonable concentrations because their combination PbI2 is insoluble. Therefore we have the correct net ionic equation and the correct prediction that Pb2+ would react with 2 I- to give PbI2 solid. A few examples will illustrate the use of the solubility rules, and the writing of net ionic equations. Example one Write the net ionic equation for any reaction that occurs when aqueous solutions of sodium carbonate and calcium perchlorate are mixed. First write all ions in solution just before any possible reaction. We have then Na+, CO3 2-, Ca2+ and ClO4- notice that this assumes that you know the formulas from the names of the compounds. Now, consult the solubility rules if necessary. If you consult the solubility rules you'll note that calcium carbonate is insoluble. Therefore, calcium 2+ and CO3 2- cannot exist in the solution together, because they must form insoluble calcium carbonate. Therefore, the net ionic equation is Ca2+ plus CO3 2- gives calcium carbonate, CaCO3. Example two Write the net ionic equation for any reaction that occurs when aqueous solutions of barium nitrate and calcium chloride are mixed. Again, write all the ions that would be in the solution just before any reaction. We have barium2+, NO3-, Ca2+ and Cl- If you consult the solubility rules you will find that all combinations of positive and negative ions are soluble. Therefore no reaction can occur.