Question:medium

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

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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.
  • Copper oxide and Copper nitrate
  • Copper hydroxide and Copper carbonate
  • Copper carbonate and Copper sulphate
  • Copper hydroxide and Copper nitrate
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The Correct Option is B

Solution and Explanation

Step 1: Note the three ingredients moist air supplies.
Air that has been sitting around a copper vessel for a long time is not just carrying oxygen, it also carries water vapour and carbon dioxide, all three of which take part in the slow surface reaction.
Step 2: Combine them with copper metal. \[ 2\text{Cu} + \text{H}_2\text{O} + \text{CO}_2 + \text{O}_2 \to \text{CuCO}_3\cdot\text{Cu(OH)}_2 \] The product written this way is really a combined salt built from two simpler compounds sitting together.
Step 3: Split the combined formula into its two components.
Reading the formula $\text{CuCO}_3 \cdot \text{Cu(OH)}_2$ carefully shows it is literally copper carbonate joined with copper hydroxide, this is the basic copper carbonate patina seen on old copper statues and vessels.
Step 4: Match with the options.
The green layer is therefore a mixture of these two named compounds together, not any single pure compound. \[ \boxed{\text{Copper hydroxide and Copper carbonate}} \]
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