Question:
Metal items are tarnished when kept open in air for some time. Silver utensils become blackish, copper utensils become greenish due to corrosion. The gases responsible for this corrosion respectively are:
Metal items are tarnished when kept open in air for some time. Silver utensils become blackish, copper utensils become greenish due to corrosion. The gases responsible for this corrosion respectively are:
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- Silver tarnishes black \(\to\) due to \(H_2S\) (forms \(Ag_2S\)).
- Copper corrodes green \(\to\) due to moist \(CO_2\) (forms basic copper carbonate).
This specific gas pairing is a standard question in board and polytechnic exams.
- Copper corrodes green \(\to\) due to moist \(CO_2\) (forms basic copper carbonate).
This specific gas pairing is a standard question in board and polytechnic exams.
Updated On: Jul 13, 2026
- \(H_2S\) and \(CO_2\)
- \(SO_2\) and \(O_2\)
- Moist \(O_2\) air and \(CO_2\)
- \(NO_2\) and \(O_2\)
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The Correct Option is A
Solution and Explanation
Step 1: Understanding the Question:
The question asks to identify the specific atmospheric gases responsible for the tarnishing/corrosion of silver objects (forming a black coating) and copper objects (forming a green coating) when exposed to air.
Step 2: Key Formula or Approach:
We write the chemical reactions representing the corrosion of silver and copper with atmospheric gases.
Step 3: Detailed Explanation:
• Tarnishing of Silver: Silver (\(Ag\)) is a relatively unreactive metal, but it reacts slowly with trace amounts of hydrogen sulfide gas (\(H_2S\)) present in the atmosphere. This reaction produces a thin, black surface layer of silver sulfide (\(Ag_2S\)):
\[ 2Ag\text{(s)} + H_2S\text{(g)} \to Ag_2S\text{(s, black)} + H_2\text{(g)} \] - Thus, the gas responsible for silver's tarnishing is hydrogen sulfide (\(H_2S\)).
• Corrosion of Copper: Copper (\(Cu\)) reacts slowly with moist carbon dioxide (\(CO_2\)), water vapor, and oxygen in the air. This process results in the formation of basic copper carbonate, which is a green mixture of copper carbonate and copper hydroxide:
\[ 2Cu\text{(s)} + H_2O\text{(l)} + CO_2\text{(g)} + O_2\text{(g)} \to CuCO_3 \cdot Cu(OH)_2\text{(s, green)} \] - Thus, the primary carbon-containing gas responsible for the green coating is carbon dioxide (\(CO_2\)).
• Matching these findings, the respective gases are \(H_2S\) and \(CO_2\).
Step 4: Final Answer:
The gases responsible are \(H_2S\) and \(CO_2\).
The question asks to identify the specific atmospheric gases responsible for the tarnishing/corrosion of silver objects (forming a black coating) and copper objects (forming a green coating) when exposed to air.
Step 2: Key Formula or Approach:
We write the chemical reactions representing the corrosion of silver and copper with atmospheric gases.
Step 3: Detailed Explanation:
• Tarnishing of Silver: Silver (\(Ag\)) is a relatively unreactive metal, but it reacts slowly with trace amounts of hydrogen sulfide gas (\(H_2S\)) present in the atmosphere. This reaction produces a thin, black surface layer of silver sulfide (\(Ag_2S\)):
\[ 2Ag\text{(s)} + H_2S\text{(g)} \to Ag_2S\text{(s, black)} + H_2\text{(g)} \] - Thus, the gas responsible for silver's tarnishing is hydrogen sulfide (\(H_2S\)).
• Corrosion of Copper: Copper (\(Cu\)) reacts slowly with moist carbon dioxide (\(CO_2\)), water vapor, and oxygen in the air. This process results in the formation of basic copper carbonate, which is a green mixture of copper carbonate and copper hydroxide:
\[ 2Cu\text{(s)} + H_2O\text{(l)} + CO_2\text{(g)} + O_2\text{(g)} \to CuCO_3 \cdot Cu(OH)_2\text{(s, green)} \] - Thus, the primary carbon-containing gas responsible for the green coating is carbon dioxide (\(CO_2\)).
• Matching these findings, the respective gases are \(H_2S\) and \(CO_2\).
Step 4: Final Answer:
The gases responsible are \(H_2S\) and \(CO_2\).
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