In the reaction between hydrogen selenide (H₂Se) and water (H₂O), H₂Se acts as the acid because it donates a proton (H⁺) to the water. The water serves as the base in this reaction because it accepts the proton. When H₂Se donates a proton, it forms the conjugate base, selenide ion (SeH⁻), while water becomes the conjugate acid, hydronium ion (H₃O⁺).
The reason H₂Se acts as an acid relates to its position in the periodic table; selenium (Se) is a chalcogen located beneath sulfur (S), which is known to form weak acids by donating protons. On the other hand, water, as a polar molecule with a bent structure and a high electronegativity of oxygen, can easily accept protons and function as a base. This interaction highlights the acid-base behavior based on the ability of substances to donate or accept protons in reactions.