Question

Brass gets discolored in the air because of the presence of which of the following gases?

• A. Oxygen
• B. Hydrogen sulfide
• C. Carbon dioxide
• D. Nitrogen

Answer 1

The correct option is B

Hydrogen sulfide

The explanation for the correct option

(B) Hydrogen sulphide

Brass

• Brass is an alloy of metallic Copper and Zinc
• The chemical composition of Brass is $66\%$ Copper and $34\%$ Zinc
• Brass is widely used in Ammunition casing, Zippers, and locks where low friction and corrosion resistance is required

The reaction of Brass with Hydrogen Sulphide $\left({\mathrm{H}}_2\mathrm{S}\right)$

• The composition of ${\mathrm{H}}_2\mathrm{S}$ in air is $<1\mathrm{ppm}$ (parts per million)
• The metallic Copper and Zinc present in Brass reacts with ${\mathrm{H}}_2\mathrm{S}$ and forms Copper sulfide$\left(\mathrm{CuS}\right)$and Zinc sulfide$\left(\mathrm{ZnS}\right)$.
• $\mathrm{CuS}$ is black which gets deposited on the surface of Brass. This leads to the decolourization of Brass.
• Brass is yellow, dull gold. When it reacts with ${\mathrm{H}}_2\mathrm{S}$ it turns black.

Hence Option (B) Hydrogen sulphide is correct

The explanation for the incorrect options

(A) Oxygen

• The composition of Oxygen $\left({\mathrm{O}}_2\right)$ in the air is $21\%$.
• Brass does not react with oxygen and thus decolourization does not occur.

(C) Carbon dioxide

• The composition of Carbon dioxide $\left({\mathrm{CO}}_2\right)$ in the air is$0.04\%$
• ${\mathrm{CO}}_2$ is not corrosive to Brass, so decolourization does not occur

(D) Nitrogen

• The composition of Nitrogen $\left({\mathrm{N}}_2\right)$ in the air is $78\%$
• ${\mathrm{N}}_2$ is an inert gas, it does not react with Brass and thus decolourization does not occur.

Therefore, the correct option is (B) Hydrogen sulphide.