When an aqueous solution containing copper and zinc ions is electrolyzed, zinc ions will remain in the solution while copper ions are reduced and discharged. Similarly, if a solution containing bromide and iodide ion is electrolyzed, iodine ions are oxidized and discharged, while bromide atoms remain in the solution. Cations can be arranged in the in the order of increasing ease of discharge at the anode. The list of ions arranged in the order of their ease of discharge at the electrodes is called Electrochemical series. If a solution contains a mixture of ions, then to find out which ion will be discharged first on electrolysis, read the electrochemical series from bottom to top, the ion at the top of the series will be discharged at the end. Sometimes there is a deviation from this principle. For example, in an aqueous solution of sodium chloride, the concentration of hydroxide ion is only 10(-7)mol.dm.(-3), whereas the concentration of chloride ion is 10(6) times times more than this. According to the electrochemical series hydrogen ion should discharge, but due to the large concentration of chloride ionspreasent in the solution, these are preferentially discharged. The electrochemical series also throw light on he reactivity of metals, which form ions in aqueous solutions.