What does acidity measure in a solution?

Acidity in a solution specifically measures the concentration of hydrogen ions (H⁺) present in that solution. The higher the concentration of hydrogen ions, the more acidic the solution becomes. This is typically quantified using the pH scale, which ranges from 0 to 14, where values below 7 indicate increasing acidity.

Understanding acidity is crucial in various environmental contexts, such as assessing water quality. For example, rainfall that has absorbed carbon dioxide from the atmosphere can form carbonic acid, leading to increased acidity in natural water bodies, which can affect aquatic life.

The other concepts represented in the other choices relate to different properties of solutions. Basicity pertains to the concentration of hydroxide ions (OH⁻), neutrality refers to a balance between H⁺ and OH⁻ ions, and viscosity concerns the thickness or resistance to flow of a liquid, which does not relate to acidity. Thus, the focus on hydrogen ion concentration firmly establishes why the measure of acidity specifically corresponds to how acidic a solution is.