When ammonium chloride (NH₄Cl) is heated, it undergoes sublimation. This means it transitions directly from a solid to a gas without passing through a liquid phase.

Here's what happens:

* Below 338°C (640°F): Ammonium chloride exists as a white crystalline solid.

* At 338°C (640°F): Ammonium chloride sublimes, meaning it vaporizes directly into gaseous ammonia (NH₃) and hydrogen chloride (HCl).

* Above 338°C (640°F): The gaseous ammonia and hydrogen chloride can recombine to form ammonium chloride again if the temperature is lowered.

However, if the heating is done in the presence of air, the decomposition products can react further:

* Ammonia (NH₃): A colorless gas with a pungent odor, it can react with oxygen in the air to form nitrogen oxides (NO_x) and water (H₂O).

* Hydrogen chloride (HCl): A colorless gas with a sharp, pungent odor, it can dissolve in water to form hydrochloric acid (HCl).

Therefore, heating ammonium chloride in the presence of air leads to the formation of a complex mixture of gases, including ammonia, hydrogen chloride, nitrogen oxides, and water vapor.

Note: The sublimation of ammonium chloride is a reversible process. If the temperature is lowered, the gaseous ammonia and hydrogen chloride can recombine to form ammonium chloride again.