What observation is made in the reaction between HCl(g) and NH₃(g)?

In the reaction between hydrogen chloride gas (HCl) and ammonia gas (NH₃), a notable observation is the formation of a white solid, which is ammonium chloride (NH₄Cl). This occurs because the HCl and NH₃ gases react to form a salt, and when they come into contact, they undergo a neutralization reaction. The resulting ammonium chloride is a solid under the conditions described.

The formation of the white solid is visually distinct and often seen as a dense white cloud or deposit in the laboratory when the gases mix. This is in contrast to the other options, which do not accurately describe the observed product of this reaction. For example, while a brown gas could suggest the presence of nitrogen dioxide or a related compound, that is not relevant in this case. If no visible change occurred, it would not reflect the actual reaction taking place. A colorless liquid formation is also not observed, as the products of this specific reaction do not include a liquid phase under typical conditions.