Question

A substance is heated with aluminium foil in aqueous sodium hydroxide. A gas is produced which turns damp, red litmus paper blue.
Which anion is present is the substance?
A. carbonate
B. iodine
C. nitrate
D. sulphate

Answer 1

Hint: Ammonia can be recognized when a pungent smell and colorless gas is released during the reaction. Here, aluminium foil acts as a strong reducing agent. Aqueous sodium hydroxide is used to increase the basicity in a solution.

Complete step by step answer:
When a substance is boiled with sodium hydroxide and fine aluminium powder or alloy, it releases fumes that have a characteristic choking smell which turns red litmus paper to blue. The gas released is ammonia. Here, fine aluminium powder acts as a powerful reducing agent that basically connects the nitrate ions into ammonia.
Therefore, the substance that is heated with aluminium foil in the aqueous solution of sodium hydroxide, is nitrate ions.

So, the correct answer is Option C.

Additional information:
Ammonia is defined as a compound that consists of nitrogen and hydrogen with a chemical formula, $N{H}_3$. It is a very common nitrogenous waste which serves as a precursor to food and fertilizers. It is used as a building block for the synthesis of many cleaning products and pharmaceutical products.

Nitrate is defined as an ion that has a formula $N{O_3}^{-}$ . It is used as a common component of explosives and fertilizers. Nitrates are soluble in water. Nitrates are used as fertilizers because of their high solubility. They are also used as oxidizing agents in which the rapid oxidation of carbon compounds liberates volume of gases.


Note: Ammonia can be characterized by a colorless gas having a very pungent smell or choking smell. Here, the aluminium foil acts as a reducing agent that converts the anion, that is, nitrate into ammonia.