* The General Form: Single replacement reactions follow the pattern: A + BC → AC + B

* In this case:

* Na₂S (aq) represents the "A" and is the sodium sulfide.

* HCl (aq) represents the "BC" and is hydrochloric acid.

* NaCl (aq) represents the "AC" and is sodium chloride.

* H₂S (g) represents the "B" and is hydrogen sulfide gas.

Explanation:

The sodium (Na) in sodium sulfide (Na₂S) replaces the hydrogen (H) in hydrochloric acid (HCl), resulting in the formation of sodium chloride (NaCl) and hydrogen sulfide gas (H₂S). This exchange of elements between two reactants characterizes a single replacement reaction.