In the given reaction, hydrogen selenide (H₂Se) acts as the acid, while water (H₂O) acts as the base. The conjugate acid formed is the hydroselenide ion (SeH⁻), and the conjugate base is the hydronium ion (H₃O⁺).
H₂Se donates a proton (H⁺) to water, making it the acid because it can release protons, representing a key characteristic of acids. Water, which accepts the proton, acts as a base. The periodic table shows that selenium (Se), being in group 16 like oxygen, shares similar chemical properties to oxoacids, and thus allows H₂Se to donate protons effectively. On the other hand, water’s ability to accept protons is common among substances with lone pairs on oxygen, allowing it to function as a base in this reaction.