The equation for the reaction between ammonia (NH₃) and hydrogen chloride (HCl) can be written as:
\[ \text{NH}_3 (g) + \text{HCl} (g) \rightarrow \text{NH}_4\text{Cl} (s) \]
In this reaction, gaseous ammonia reacts with gaseous hydrogen chloride to form solid ammonium chloride (NH₄Cl). This is an example of an acid-base reaction where ammonia acts as a base and hydrogen chloride acts as an acid.