Final answer:
The claim that groundwater is comprised mostly of H2O ions is false; most water exists as H2O molecules. It is the polarity of water that contributes to its solvent properties, not the concentration of H2O ions.
Step-by-step explanation:
The statement that groundwater is made up of mostly H2O ions is false. In water, a very small fraction of water molecules dissociate into hydronium (H3O+) and hydroxide (OH−) ions, but most of the water exists as H2O molecules. It is the polar nature of the water molecule that makes it an excellent solvent for many substances, including organic materials. Groundwater indeed contains many dissolved substances, such as minerals and ions, from its contact with soil and rocks. These ions can affect the pH of the water, making it more acidic or basic but do not make up the majority of the water's composition.