Ammonium chloride NH₄Cl is, in its pure form, a clear white water-soluble crystalline salt of ammonia. The aqueous ammonium chloride solution is mildly acidic. The substance occurs naturally in volcanic regions, forming on volcanic rocks near fume-releasing vents. The crystals deposit directly from the gaseous state, and tend to be short-lived, as they dissolve easily in water. It is a by-product of the Solvay process used to produce sodium carbonate.

Ammonium chloride is prepared commercially by reacting ammonia (NH₃) with hydrogen chloride (HCl). As these chemicals are corrosive, this process has to be performed in vessels lined with nonreactive materials.

NH₃ + HCl → NH₄Cl

This reaction can occur if poorly sealed bottles of household ammonia and hydrochloric acid are stored in close proximity, leading to crystals forming around the openings of the bottles.