Final answer:
Carbonation is the process that causes pure water's pH to drop below neutral when exposed to the atmosphere, due to CO2 forming carbonic acid which releases H+ ions and lowers pH.
Step-by-step explanation:
The process that will cause the pH of pure water exposed to the atmosphere to drop below neutral is carbonation. This phenomenon occurs when carbon dioxide (CO2) from the atmosphere dissolves in water and forms carbonic acid (H2CO3), which then dissociates into bicarbonate (HCO3-) and hydrogen (H+) ions. The increase in H+ concentration leads to a decrease in the pH of the water, making it more acidic.
When atmospheric CO2 levels rise, more CO2 is absorbed into ocean waters, leading to ocean acidification. Acid molecules like carbonic acid added to water will release protons, driving the formation of H3O+ ions in the solution, and thus reducing the pH level below 7, the neutrality point of water.