In the reaction between hydrogen selenide (H2Se) and water (H2O), H2Se acts as the acid while water acts as the base. This is because H2Se donates a proton (H⁺) to water, which makes water a proton acceptor. The resulting species, SeH, is the conjugate base of H2Se, while H3O⁺ (hydronium ion) is the conjugate acid formed from water accepting the proton.
Hydrogen selenide is able to function as an acid due to the presence of the relatively electronegative selenium atom, which makes the hydrogen atoms more positive and more likely to be donated as protons. Water (H2O), a polar molecule with hydrogen bonding capabilities, can readily accept protons due to its strong affinity for protons, thus acting as a base in this chemical reaction.