In the given reaction, hydrogen selenide (H₂Se) acts as the acid, while water (H₂O) acts as the base. The conjugate acid is SeH¯, and the conjugate base is H₂O*.
Hydrogen selenide (H₂Se) acts as the acid because it donates a proton (H⁺) to water. Water (H₂O) acts as the base because it accepts the proton from hydrogen selenide.
The tendency for a substance to act as an acid or base can be explained using patterns in the periodic table. In this case, hydrogen selenide (H₂Se) is an acid because it contains hydrogen (H) bonded to a more electronegative element, selenium (Se). Water (H₂O) acts as a base because oxygen (O) is more electronegative than hydrogen (H), making it more likely to accept a proton.