Here's why:

* Ammonia is a weak base. It can accept a proton (H⁺) from an acid.

* Acids donate protons.

* The reaction is a neutralization reaction. The base and acid neutralize each other.

Example:

When ammonia reacts with hydrochloric acid (HCl), ammonium chloride (NH₄Cl) and water (H₂O) are formed:

NH₃(aq) + HCl(aq) → NH₄Cl(aq) + H₂O(l)

General Equation:

NH₃(aq) + HX(aq) → NH₄X(aq) + H₂O(l)

where HX is any acid.