Final answer:
A water molecule loses a proton to form the hydroxide ion OH-. This results in the formation of a hydroxide ion and a hydronium ion, representing conjugate base and acid respectively in the self-ionization of water.
Step-by-step explanation:
The molecule that loses a proton to form the hydroxide ion OH- is a water (H2O) molecule. When water loses a proton (H+), the remaining molecule becomes a hydroxide ion with a negative charge. As part of the self-ionization of water, this process also results in the formation of a hydronium ion (H3O+), which occurs when another water molecule accepts the proton that was lost.
The reaction can be represented as follows:
2H2O → H3O+ + OH-
Conjugate acid-base pairs are relevant in this context where the water molecule that donates a proton (acting as a Brønsted-Lowry acid) converts to its conjugate base OH-, and the molecule that accepts a proton (acting as a Brønsted-Lowry base) becomes the conjugate acid H3O+. This equilibrium reaction is reversible and essential to understanding the pH of solutions.