Final answer:
An amphoteric molecule paired with a base can act as an acid. This is because an amphoteric, or amphiprotic, substance has the ability to either donate or accept a proton, depending on the reaction conditions and the reactants present.
Step-by-step explanation:
An amphoteric molecule, when paired with a base, can act as an acid. The term amphoteric refers to the ability of a substance to behave as either an acid or a base. An amphoteric substance like water can either donate a proton and act as an acid, or accept a proton and act as a base, depending on the other reactants present in the reaction. This characteristic is also known as amphiprotic behavior. For example, water can act as an acid when it donates a proton to ammonia (NH3), forming the ammonium ion (NH4+) and a hydroxide ion (OH-). Conversely, water acts as a base when it accepts a proton from hydrochloric acid (HCl), forming the hydronium ion (H3O+) and a chloride ion (Cl-).