Which gas turns damp red litmus paper blue?

Which gas turns damp red litmus paper blue?

• A. Ammonia
• B. Carbon Dioxide
• C. Oxygen
• D. Hydrogen

Answer: (A)

Gas that turns damp red litmus paper blue is ammonia because it behaves as a base. When ammonia dissolves in water, it reacts to form hydroxide ions: NH3 + H2O ⇌ NH4+ + OH−. The presence of OH− makes the solution alkaline, and alkaline (basic) conditions turn damp red litmus blue. The other gases don’t produce alkaline solutions. Carbon dioxide makes an acidic solution in water (CO2 + H2O ⇌ H2CO3), which would not turn red litmus blue. Oxygen and hydrogen don’t create bases in this context, so they don’t change damp red litmus blue.

Gas that turns damp red litmus paper blue is ammonia because it behaves as a base. When ammonia dissolves in water, it reacts to form hydroxide ions: NH3 + H2O ⇌ NH4+ + OH−. The presence of OH− makes the solution alkaline, and alkaline (basic) conditions turn damp red litmus blue.

The other gases don’t produce alkaline solutions. Carbon dioxide makes an acidic solution in water (CO2 + H2O ⇌ H2CO3), which would not turn red litmus blue. Oxygen and hydrogen don’t create bases in this context, so they don’t change damp red litmus blue.