Final answer:
HF, or hydrofluoric acid, is the compound that behaves as an acid when dissolved in water. Water can act as either an acid or a base in chemical reactions, but pure water is neutral because it has equal concentrations of H3O+ and OH- ions.
Step-by-step explanation:
The compound that behaves as an acid when dissolved in water among BaO, C₅H₁₂, HF, and NaOH is HF (hydrofluoric acid). When an acid dissolves in water, it donates a proton (H+) to the water, forming hydronium ions (H3O+). For instance, in the Brønsted-Lowry acid-base theory, acids are proton donors, and bases are proton acceptors. Therefore, HF, being able to release H+ ions in an aqueous solution, behaves as an acid.
When considering exercise questions where water might act as an acid or a base, it's important to look at what is happening in the chemical reaction. For example, when NH4+ (aq) reacts with H2O, water acts as a base because it accepts a proton from NH4+. Conversely, water can act as an acid, as in the reaction where H2O (l) + NH3 (aq) → H³O*(aq) + NH3 (aq), where water donates a proton. Pure water is considered neutral because it has an equal concentration of hydronium ions (H3O+) and hydroxide ions (OH−), which balance out to make a neutral solution.