Ace Aqueous Adventures: NCEA Level 3 Chemistry (AS91393) Practice Exam 2025 – Dive In and Dominate!

An Arrhenius acid is defined as a substance that increases the concentration of hydrogen ions (H+) in an aqueous solution. When an Arrhenius acid dissolves in water, it dissociates to release H+ ions, which is a key characteristic of acids according to Arrhenius's theory. This characteristic is pivotal in understanding acid-base reactions within aqueous solutions.

For example, hydrochloric acid (HCl) dissociates in water to produce H+ ions and chloride ions (Cl-), thus increasing the concentration of H+ ions in the solution. This increase in H+ concentration is what defines its acidic properties and its behavior as an Arrhenius acid.

In contrast, a substance that produces hydroxide ions in solution is classified as an Arrhenius base, not an acid. Similarly, a substance that decreases the concentration of H+ ions would not fit the Arrhenius definition, as it would be acting as a base rather than an acid. A neutral substance remains unchanged in its ion concentration in solution and does not contribute to the acidic properties defined by Arrhenius acids.