To detect nitrate ions, which procedure will produce ammonia when warmed?

• A. Add aqueous sodium hydroxide, aluminium foil and warm carefully.
• B. Add concentrated sulfuric acid and heat.
• C. Add calcium chloride and boil gently.
• D. Pass nitrogen gas through the solution.

Answer: (A)

Nitrate ions can be detected by reducing them to ammonia under strongly basic, reducing conditions. In this test, adding sodium hydroxide creates a highly alkaline environment, and the aluminium foil provides a reducing agent. When the mixture is warmed, the aluminium reduces the nitrate ions to ammonia, which escapes as a gas. The ammonia then turns damp red litmus blue, giving a clear positive result.

Other procedures don’t create the right conditions or produce ammonia. Heating with concentrated sulfuric acid tends to generate nitrogen oxides rather than ammonia. Using calcium chloride and boiling won’t drive the reduction to ammonia, and passing nitrogen gas through the solution doesn’t generate ammonia either.