Brass gets discoloured in air due to the presence of which gas?

Brass is an alloy made mainly of copper (Cu) and zinc (Zn). When brass objects are exposed to the atmosphere for a long period, they often lose their bright golden shine and develop a dull or dark coating. This discoloration is primarily caused by the presence of hydrogen sulphide (H₂S) gas in the air.

Hydrogen sulphide is a trace gas that can be found in the environment, particularly in polluted or industrial areas. It is produced from the decay of organic matter, sewage, and certain industrial emissions. Even though it is present in very small quantities, H₂S reacts readily with copper, one of the major constituents of brass.

The chemical reaction that takes place is:
Cu + H₂S → CuS + H₂

In this reaction, copper (Cu) combines with hydrogen sulphide (H₂S) to form copper sulphide (CuS), which is black in color. This black layer of copper sulphide forms on the surface of brass articles, giving them a tarnished appearance. Over time, this tarnish can become thicker and dull the metallic luster completely.

Zinc, the other component of brass, reacts much less readily with hydrogen sulphide, so the discoloration mainly results from the reaction of copper. The process is an example of surface corrosion caused by environmental gases.

Prevention Methods:

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  Applying a thin coat of lacquer or polish on brass surfaces can prevent contact with air and H₂S gas.

  
  


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  Keeping brass items in dry, low-sulphur environments reduces tarnishing.

  
  


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  Regular cleaning with mild acidic solutions (like lemon juice or vinegar) helps remove the oxide and sulphide layers.

  
  

Conclusion:
The discoloration of brass in air is due to the reaction of hydrogen sulphide gas with copper, forming black copper sulphide (CuS). This is a classic example of how metals and alloys react chemically with atmospheric gases, resulting in surface tarnish or corrosion.