Which species has a low concentration of hydroxide ions?

An acidic solution has a low concentration of hydroxide ions. In the context of acid-base chemistry, acids produce hydrogen ions (H⁺) when dissolved in water, which leads to an increase in the H⁺ concentration. According to the relationship between hydrogen ions and hydroxide ions, the greater the concentration of H⁺, the lower the concentration of OH⁻, since water dissociates to maintain the equilibrium of H⁺ and OH⁻ ions.

In acidic conditions, this equilibrium shifts, resulting in fewer hydroxide ions present. This is what characterizes an acid compared to alkaline (basic) solutions, where the concentration of hydroxide ions is higher, and neutral solutions, which have equal concentrations of H⁺ and OH⁻ ions. Therefore, recognizing how the pH scale relates to the concentrations of these ions clarifies why an acidic solution specifically has a low concentration of hydroxide ions.