Final answer:
The lone hydrogen ion (H+) combines with an un-ionized water molecule to form a hydronium ion (H3O+), while the donating water molecule becomes a hydroxide ion (OH-) through the process of self-ionization.
Step-by-step explanation:
When a lone hydrogen ion (H+), which is essentially a proton due to the absence of its electron, encounters a water molecule (H2O), it forms a hydronium ion (H3O+). This occurs because the proton is highly attracted to the negative charge on the oxygen atom's lone pair of electrons in the water molecule. Consequently, a coordinate bond is created between the proton and the water molecule, resulting in the hydronium ion.
Additionally, this process accompanies the generation of a hydroxide ion (OH-), which maintains equilibrium through the self-ionization of water:
2H2O → H3O+ + OH-
In essence, water acts as both an acid and a base in this transfer, wherein one water molecule donates a proton becoming an OH- ion, and the other accepts the proton to become H3O+ ion. This fundamental reaction is key to the chemistry of acids and bases in aqueous solutions, emphasizing that there are no truly free protons in such an environment; instead, these protons are associated with water molecules.