Suppose you have an aqueous solution containing Cu \(^{2+}\) ions and \(\mathrm{Na}^{+}\) ions. What could you add that would remove the \(\mathrm{Cu}^{2+}\) ions from solution without adding any new type of cations to the solution?

In order to find a substance that can selectively react with Cu²⁺ ions without adding any new type of cations, we need to recall the solubility rules for ionic compounds. The important rules relevant to this exercise are: 1. Most nitrate (NO₃⁻) salts are soluble. 2. Most salts containing the alkali metal ions (such as Na⁺) and the ammonium ion (NH₄⁺) are soluble. 3. Most chloride, bromide, and iodide salts are soluble, except for those of silver (Ag⁺), mercury(I) (Hg₂²⁺), and lead (Pb²⁺). 4. Most sulfate (SO₄²⁻) salts are soluble, except for those of calcium (Ca²⁺), strontium (Sr²⁺), barium (Ba²⁺), and lead (Pb²⁺). 5. Most hydroxide (OH⁻) salts are only slightly soluble. The alkali metal hydroxides and the heavier alkaline earth metal hydroxides (such as Ca(OH)₂, Sr(OH)₂, and Ba(OH)₂) are more soluble.

To selectively remove Cu²⁺ ions from the solution without adding any new type of cations, we can look for an anion that forms an insoluble compound with Cu²⁺ but remains soluble with Na⁺ ions. Based on the solubility rules, we can choose hydroxide ions (OH⁻) since they form an insoluble compound with Cu²⁺ ions but remain soluble with Na⁺ ions. Therefore, we can add sodium hydroxide (NaOH) to the solution, which will provide hydroxide ions (OH⁻) without adding any new type of cations (since Na⁺ is already present in the solution).

When sodium hydroxide is added to the aqueous solution containing Cu²⁺ ions and Na⁺ ions, the following chemical reaction occurs: \(Cu^{2+}(aq) + 2OH^-(aq) \longrightarrow Cu(OH)_2(s)\) Copper(II) hydroxide, Cu(OH)₂, is an insoluble compound, which will precipitate out of the solution. This will remove Cu²⁺ ions from the solution.

To remove Cu²⁺ ions from an aqueous solution containing Cu²⁺ and Na⁺ ions without adding any new type of cations, we can add sodium hydroxide (NaOH). This will selectively react with Cu²⁺ ions to form an insoluble compound, copper(II) hydroxide Cu(OH)₂, which can then be separated from the solution through filtration or centrifugation.