Final answer:
Among the choices, BF₃ is identified as the Lewis acid because it can accept a pair of electrons due to boron's electron-deficient valence shell.
Step-by-step explanation:
To identify the Lewis acid among the given compounds, we must understand that a Lewis acid is a chemical species that can accept a pair of electrons. Among the options provided (HCl, H₂O, NH₃, and BF₃), BF₃ is the Lewis acid because boron has only six electrons in its valence shell, making it electron-deficient and capable of accepting a pair of electrons to complete its octet.
HCl is a strong Brønsted-Lowry acid as it donates a proton to substances like water. This is evident when HCl reacts with NH₃, where HCl donates a proton to NH₃. Water, or H₂O, can act as both a Brønsted-Lowry acid and a Brønsted-Lowry base, depending on the reaction it participates in. For example, in the reaction with NH₄+, water acts as a base by accepting a proton. NH₃ (ammonia) is typically a Brønsted-Lowry base as it accepts protons from acids like HCl.