The two test tubes on the left contain solutions of cupric chloride. The red-brown material in the second test tube from the right is a precipitate of silver chromate. The test tube on the far right contains a solution of sodium carbonate. A solution of sodium sulfate is added to the first test tube on the far left. There is no visible reaction because neither of the two possible products--cupric sulfate and sodium chloride--are insoluble or weak electrolytes. When a solution of sodium carbonate is added to the other solution of cupric chloride, insoluble cupric carbonate is formed. The addition of colorless barium nitrate to the next test tube results in the formation of yellow barium chromate. In this ion combination reaction, the less soluble barium chromate is formed at the expense of the more soluble silver chromate. Finally, a solution of hydrochloric acid is added to the sodium carbonate solution. The hydronium ions in the acid solution react with the carbonate ions to form carbonic acid, a weak electrolyte, which then decomposes to carbon dioxide. The carbon dioxide can be seen fizzing off as the acid is added.
A saturated solution of potassium iodide is added to a solution of lead nitrate. Initially, a yellow precipitate of lead iodide is formed, but continued addition of the iodide ion leads to the dissolution of the lead iodide and formation of the soluble K2PbI4 salt. The two reactions are an ion-combination reaction and an electron-sharing (Lewis acid-base) reaction, respectively.