Final answer:
A water molecule that gains a hydrogen ion becomes a hydronium ion (H3O+), which occurs in a reversible reaction with hydroxide ions forming in aqueous solutions.
Step-by-step explanation:
When a water molecule gains a hydrogen ion, it becomes a hydronium ion (H3O+). This occurs because hydrogen ions (H+) tend to bond with H2O molecules in an aqueous solution, as separate hydrogen ions do not exist freely in such an environment.
The reaction can be represented as 2H2O → H3O+ + OH−, where one water molecule acts as a Brønsted-Lowry base by accepting a proton to become H3O+ and the other as an acid by donating a proton to form a hydroxide ion (OH−).
Water's polar nature, with a partial negative charge due to its bent molecular geometry and oxygen's higher electronegativity, enables it to attract H+ ions and form the hydronium ion.
This process is an example of a reversible reaction, highlighting the dynamic equilibrium between the dissociation of water molecules and the reformation of water from hydronium and hydroxide ions.