part a is an oxidation-reduction reaction in which metallic copper,Cu(s),is oxidized by the NO3 ion from HNO3 to give a blue aqueous solution of Cu2+ ions. The NO3 ions are simultaneously reduced to a brown gas,NO2 (EP.1,1a and 1b). (NOTE: "a" equations are total ionic and "b" equations are net ionic.)
Although the formulas for ions in aqueous solution are usually represented as simple ions,this is an oversimplification. In aqueous solution, as well as in many solids, ions are hydrated, i.e., wathe molecules are attached to the ions. The exact degreehydration in agueous solution is known for relatively few ions. Copper(II)ions usually exist in the solid state as[Cu(OH2)4]2+ ions. H2) is written OH2 to indicate Cu-O bonds. Compounds such as copper(II)nitrate and copper(II)sulfate would be sources of Cu2+ ions. The blue color of dilute aqueous solutions of copper(II)nitrate and copper(II)sulfate are blue,indicates that [Cu(OH2)4]2+ ions probably do not exist in solution of copper(II)chloride. Or,if [Cu(OH2)4]2+ ions do exist,some other colored species must also be presert in the solution to cause the color to be green rather than blue.It is generally accepted that the green color of copper(II)chloride solution is due to [Cu(OH2)4-n(Cl n](2-n). In these species the number of chloride ions,"n," may vary from one to four depending on the concentration of chloride ions in the solution.