American Chemical Society (ACS) Chemistry Practice Exam – Practice Test & Study Guide

The correct answer is that acids increase the concentration of H+ ions in a solution. When an acid dissolves in water, it dissociates, releasing protons (H+) into the solution. This increase in H+ concentration is what characterizes the acidic nature of the solution, leading to a lower pH value.

In water, the presence of these H+ ions drives the equilibrium of various reactions and can influence properties such as the chemical behavior of the solution and its interactions with other substances. For instance, an increase in H+ ions can enhance the ability of the solution to conduct electricity and can also participate in various chemical reactions, particularly those involving acid-base chemistry.

The other options do not reflect what acids contribute to a solution. The concentration of H2O remains relatively unchanged in a dilute aqueous solution of an acid, while the concentration of OH- ions typically decreases due to the common ion effect, which is a result of an increased concentration of H+ ions. The presence of CO2 is not directly relevant in the context of acid dissociation in solutions.