Question

When a copper vessel is exposed to moist air for long, it acquires a dull green coating. The green material is a mixture of:

• A. Copper oxide and Copper nitrate
• B. Copper hydroxide and Copper carbonate
• C. Copper carbonate and Copper sulphate
• D. Copper hydroxide and Copper nitrate

Hint:

The corrosion of copper produces "basic copper carbonate" which is a 1:1 mixture of copper hydroxide and copper carbonate:
\[ \text{Cu(OH)}_2 + \text{CuCO}_3 \]
Remember this as the reason behind the green color of old copper objects.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the chemical composition of the dull green coating (patina) that forms on the surface of a copper object when it undergoes corrosion upon long exposure to moist air.

Step 2: Key Formula or Approach:
Corrosion of copper involves reaction with atmospheric water, carbon dioxide, and oxygen:
\[ 2\text{Cu} + \text{H}_2\text{O} + \text{CO}_2 + \text{O}_2 \to \text{CuCO}_3\cdot\text{Cu(OH)}_2 \]

Step 3: Detailed Explanation:

• When copper is exposed to moist air containing oxygen, carbon dioxide, and water vapor over a long period, it slowly corrodes.

• The copper surface reacts with these gases to form a green protective layer called patina.

• This green coating consists of basic copper carbonate, which is a mixture of copper carbonate (\(\text{CuCO}_3\)) and copper hydroxide (\(\text{Cu(OH)}_2\)).

• The balanced chemical equation representing this slow corrosion is:
\[ 2\text{Cu(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} + \text{O}_2\text{(g)} \to \text{CuCO}_3 \cdot \text{Cu(OH)}_2\text{(s)} \] - This chemical compound gives old copper monuments their characteristic green color.


Step 4: Final Answer:
The green coating is a mixture of copper hydroxide and copper carbonate.