Final answer:
Adding NH₃ to a CuSO₄ solution increases the solubility of Cu²+ by forming a complex ion, turning the solution a deep blue color. When HCl is added, it reduces the availability of NH₃ for complex formation with Cu²+, potentially causing the solution's color to fade.
Step-by-step explanation:
When NH₃ (ammonia) is added to a CuSO₄ (copper(II) sulfate) solution, it increases the solubility of Cu²+, as ammonia forms a complex ion with Cu²+. According to Le Chatelier's principle, adding NH₃ will shift the equilibrium to the right, promoting the formation of the [Cu(NH₃)4]²+ complex ion. This reaction is shown as Cu(NO₃)2 + 4NH₃ + 6H₂O → “[Cu(NH₃)4]H₂O”(NO₃)₂.
The formation of this complex causes a deep blue color to appear in the solution, and since the equilibrium constant for the reaction is large, this reaction lies far to the right, indicating that almost all of the copper ions will react with ammonia.
Conversely, when HCl is added to the solution, it introduces Cl⁻ ions and H⁺ ions. The increased concentration of H⁺ ions can react with the NH₃ in solution to form NH₄⁺ and Cl⁻, thus reducing the ammonia available to form the copper-ammonia complex. As a result, the equilibrium will shift to the left to minimize this change, and the color of the solution may fade as the complex ion concentration decreases.