What is the test for ammonium ions?

What is the test for ammonium ions?

• A. Add acid to solution; limewater turns cloudy.
• B. Add sodium hydroxide and warm; ammonia produced turns damp red litmus blue.
• C. Add barium chloride; white precipitate forms.
• D. Add AgNO3; yellow precipitate forms.

Answer: (B)

Ammonium ions are detected by releasing ammonia gas when they react with a strong base and heat. The reaction NH4+ + OH− → NH3 + H2O shows that a base converts the ammonium ion into ammonia. Warming speeds this up, so more NH3 gas is produced. Ammonia is alkaline, so it raises the pH of its surroundings; when it comes into contact with damp red litmus paper, the paper turns blue. This blue change confirms the presence of ammonium ions. The other tests look for different ions: one identifies carbonate by producing CO2 that turns limewater cloudy, another detects sulfate with a white precipitate of barium sulfate, and the last detects halides with a silver salt precipitate. None of these involve releasing ammonia gas or changing litmus, so they wouldn’t indicate ammonium ions.

Ammonium ions are detected by releasing ammonia gas when they react with a strong base and heat. The reaction NH4+ + OH− → NH3 + H2O shows that a base converts the ammonium ion into ammonia. Warming speeds this up, so more NH3 gas is produced. Ammonia is alkaline, so it raises the pH of its surroundings; when it comes into contact with damp red litmus paper, the paper turns blue. This blue change confirms the presence of ammonium ions.

The other tests look for different ions: one identifies carbonate by producing CO2 that turns limewater cloudy, another detects sulfate with a white precipitate of barium sulfate, and the last detects halides with a silver salt precipitate. None of these involve releasing ammonia gas or changing litmus, so they wouldn’t indicate ammonium ions.