The diffusion of aqueous ammonia (NH₃) and concentrated hydrochloric acid (HCl) results in a chemical reaction producing ammonium chloride (NH₄Cl), a white solid that appears as white fumes.

Here's a breakdown of what happens:

1. Diffusion: Both ammonia and hydrochloric acid are gases that dissolve in water. When they are in close proximity, their molecules diffuse, moving from areas of high concentration to areas of low concentration.

2. Reaction: Ammonia (NH₃) is a base and hydrochloric acid (HCl) is an acid. When they come into contact, they react in a neutralization reaction:

```

NH<sub>3</sub>(g) + HCl(aq) → NH<sub>4</sub>Cl(s)

```

3. Formation of Ammonium Chloride: The reaction produces ammonium chloride (NH₄Cl), a white solid. This solid is initially formed as tiny particles, which appear as white fumes because they are suspended in the air.

4. Dissolution: Over time, the ammonium chloride particles will dissolve in the water, forming a solution.

Visualizing the Reaction:

You can observe this reaction by holding a glass rod dipped in concentrated hydrochloric acid near a container of aqueous ammonia. The white fumes of ammonium chloride will be readily visible, signifying the reaction taking place.

Important Note: This reaction is exothermic, meaning it releases heat. It can be dangerous if not handled carefully. Always work in a well-ventilated area and avoid direct contact with the chemicals.