To determine when we observe a blue color in the context of a copper (Cu) electrode, we need to consider the chemical reactions that occur when the electrode is placed in various solutions. The blue color is primarily associated with the presence of copper ions in solution, particularly in the form of copper(II) ions (Cu²⁺). Let's analyze each option provided to see which one leads to the observation of a blue color.
Analyzing Each Solution
1. Cu Electrode in AgNO₃ Solution
When a copper electrode is placed in silver nitrate (AgNO₃) solution, a displacement reaction occurs. Copper is more reactive than silver, so it displaces silver ions from the solution, leading to the formation of solid silver and copper ions in the solution. However, the solution itself does not turn blue; instead, you would see a silver precipitate forming.
2. Cu Electrode in ZnSO₄ Solution
In a zinc sulfate (ZnSO₄) solution, no significant reaction occurs because zinc is more reactive than copper. The copper electrode remains unchanged, and the solution does not develop a blue color. Therefore, this option does not lead to the observation of blue color either.
3. Cu Electrode in Dilute HNO₃ Solution
When a copper electrode is placed in dilute nitric acid (HNO₃), a reaction occurs where copper is oxidized to copper ions (Cu²⁺), which are blue in color. The nitric acid acts as an oxidizing agent, and the resulting copper ions in the solution give it a distinct blue hue. This is the first instance where we can observe a blue color.
4. Cu Electrode in Dilute H₂SO₄ Solution
In dilute sulfuric acid (H₂SO₄), a similar reaction occurs as with nitric acid. The copper electrode oxidizes, producing copper ions (Cu²⁺) in the solution, which again results in a blue color. Thus, this option also leads to the observation of blue color due to the presence of Cu²⁺ ions.
Summary of Findings
- AgNO₃ solution: No blue color observed.
- ZnSO₄ solution: No blue color observed.
- Dilute HNO₃ solution: Blue color observed due to Cu²⁺ ions.
- Dilute H₂SO₄ solution: Blue color observed due to Cu²⁺ ions.
In conclusion, the blue color is observed when the copper electrode is placed in either dilute nitric acid or dilute sulfuric acid solutions, as both lead to the formation of copper(II) ions in the solution. Therefore, the correct answers are options (c) and (d).
