The concentration of which ion determines if a solution is acidic?

In an aqueous solution, the concentration of the hydronium ion (H₃O⁺) fundamentally determines whether the solution is acidic. A solution is classified as acidic when the concentration of hydronium ions exceeds that of hydroxide ions (OH⁻). According to the Brønsted-Lowry theory, acids are substances that donate protons (H⁺ ions), which in water results in an increase in hydronium ions.

The pH scale, which measures the acidity or basicity of a solution, is logarithmically related to the concentration of hydronium ions. A pH less than 7 indicates an acidic solution, which corresponds to a higher concentration of hydronium ions. Thus, a clear and direct relationship exists between hydronium ion concentration and the acidity of the solution, reinforcing that it is the hydronium ion that is the key factor in determining acidity.