Brass gets discoloured or tarnished in air mainly because of the presence of which gas, which reacts to form a surface coating?

Introduction / Context:
Brass is an alloy of copper and zinc and is widely used in decorative items, utensils, and fittings. Over time, brass articles tend to lose their shine and develop a discoloured surface. This tarnishing is a common observation in everyday life and can be explained using basic chemistry. This question asks which gas in the air is primarily responsible for the discolouration of brass, testing your understanding of corrosion and surface reactions.

Given Data / Assumptions:
- The metal alloy in question is brass, containing copper and zinc. - The discolouration occurs when brass is exposed to air over time. - Options include hydrogen sulphide, oxygen, nitrogen, and carbon dioxide. - We assume typical atmospheric composition with trace gases present.

Concept / Approach:
Copper and its alloys, like brass, are sensitive to sulphur-containing gases. Hydrogen sulphide present in small amounts in the atmosphere can react with copper in brass to form copper sulphide, a dark compound that leads to tarnishing. Oxygen and carbon dioxide do play roles in general corrosion, but the characteristic dark discolouration is mainly due to sulphide formation. Nitrogen is relatively inert under normal conditions and does not significantly react with brass. The approach is to link the observed discolouration with the formation of sulphide layers from hydrogen sulphide in the air.

Step-by-Step Solution:
Step 1: Recall that brass contains copper, which readily reacts with sulphur compounds in the environment. Step 2: Hydrogen sulphide (H2S) is a trace gas in the atmosphere, often arising from decaying organic matter and industrial pollution. Step 3: When brass is exposed to air containing H2S, a chemical reaction occurs forming copper sulphide on the surface, which is dark and causes discolouration. Step 4: Oxygen alone tends to form oxides, which may also dull the surface, but the specific tarnishing often associated with brass and silver is linked to sulphides. Step 5: Nitrogen is largely inert at ordinary temperatures and does not react significantly with metals like copper under normal atmospheric conditions. Step 6: Carbon dioxide can react with metal oxides to form carbonates in some cases, but it is not the primary cause of the characteristic sulphide tarnish on brass. Step 7: Therefore, hydrogen sulphide is the gas most directly responsible for discolouring brass in air.

Verification / Alternative check:
You can verify this by recalling that silver jewellery and cutlery also tarnish in the presence of hydrogen sulphide, forming black silver sulphide. Similar chemistry applies to the copper component of brass. Real-world cleaning products for brass and silver often advertise their ability to remove sulphide tarnish, further confirming that sulphur-containing gases are the main culprits rather than oxygen or carbon dioxide alone.

Why Other Options Are Wrong:
Oxygen (option b) does promote oxidation, but the dull oxide film it forms is generally different in colour and does not fully explain the characteristic dark tarnish associated with brass. Nitrogen (option c) is very stable as N2 and typically does not react with metals in ordinary air. Carbon dioxide (option d) is involved in the formation of carbonates in some metals but is not the main factor causing the specific discolouration observed in brass due to sulphide formation. Therefore, these gases are less significant than hydrogen sulphide in tarnishing brass.

Common Pitfalls:
A common mistake is to assume that oxygen must be responsible for all forms of corrosion and discolouration because rusting of iron is widely known as an oxidation process. However, for alloys like brass and metals like silver, sulphur-containing gases such as hydrogen sulphide are the major causes of tarnish. Remembering this difference between oxide formation and sulphide tarnish will help you answer such questions correctly.

Final Answer:
Brass gets discoloured in air mainly because of the presence of hydrogen sulphide, which forms dark sulphide coatings on its surface.