In the reaction between hydrogen selenide (H2Se) and water, H2Se acts as the acid, while water (H2O) functions 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, which accepts it, reflecting the typical behavior of acids and bases as defined by the Brønsted-Lowry theory.
In the periodic table, hydrogen selenide is related to other hydrogen halides like HCl and HBr, which are known acids; as we move down the group, the bond polarity and ability to donate protons increase due to the larger atomic size and lower electronegativity of the heavier elements. Additionally, water's high electronegativity allows it to act as an effective base by accepting protons, reinforcing the role of these substances in the acid-base reaction.