Pure water is considered neutral because it has a balanced pH level of 7. This neutrality is due to the equal concentration of hydrogen ions (H+) and hydroxide ions (OH–) in the solution.
In pure water, a small number of water molecules dissociate into H+ and OH– through a process called self-ionization. This reaction can be represented as:
2 H2O ⇌ H+ + OH–
At 25 degrees Celsius (room temperature), the concentration of these ions in pure water is equal, typically around 1 x 10-7 moles per liter for both H+ and OH–. Because the concentrations of hydrogen and hydroxide ions are the same, the solution is balanced, which is why water has a pH of 7.
When the pH is below 7, it indicates an acidic solution (more H+ than OH–), and when the pH is above 7, it indicates a basic solution (more OH– than H+). However, since pure water maintains an equal concentration of these ions, it remains neutral.