At 25 °C, an acidic aqueous solution has a hydrogen-ion concentration greater than its hydroxide-ion concentration.

### Why this is right An acidic solution has more hydrogen ions than hydroxide ions. At 25 °C, neutral water has equal concentrations of the two ions, so an excess of hydrogen ions gives a pH below 7. ### The concept Acidity is defined by relative ion concentrations, not by whether the solution has a net electrical charge. A solution remains electrically neutral overall because its positive and negative charges balance. ### Terms - **Hydroxide ion**: the negatively charged ion OH⁻. - **Aqueous**: dissolved in water. ### Remember At 25 °C, acid means $[H^+] > [OH^-]$ and pH below 7.