Aqueous solution of ammonia gives a pungent smell due to the presence of dissolved ammonia gas. When ammonia gas is dissolved in water, it undergoes a chemical reaction with water molecules to form ammonium hydroxide (NH4OH). This reaction is reversible, meaning that ammonium hydroxide can also decompose back into ammonia gas and water. The equilibrium between these two reactions is temperature-dependent, with more ammonia gas being dissolved at lower temperatures.

The pungent smell of aqueous ammonia is due to the fact that ammonia gas is a strong irritant to the mucous membranes of the nose, throat, and eyes. When ammonia gas is inhaled, it can cause a burning sensation, coughing, and difficulty breathing. In severe cases, ammonia gas can also cause pulmonary edema, a condition in which fluid accumulates in the lungs.

The pungent smell of aqueous ammonia is also a warning sign of its potential toxicity. Ammonia is a corrosive substance that can cause severe burns to the skin and eyes. It is also a flammable gas, and can form explosive mixtures with air.

For these reasons, it is important to take precautions when working with aqueous ammonia. Always wear gloves, eye protection, and a respirator when handling this substance. If you spill aqueous ammonia on your skin or in your eyes, flush the affected area with water for at least 15 minutes. If you inhale ammonia gas, move to a fresh air area and seek medical attention immediately.