The reason pure water is considered neutral with respect to pH is that it has an equal concentration of hydrogen ions (H+) and hydroxide ions (OH-). The pH scale measures the acidity or basicity of a solution on a scale from 0 to 14, with 7 being neutral. When the concentration of H+ ions is greater than the concentration of OH- ions, the solution is acidic and has a pH value less than 7. Conversely, when the concentration of OH- ions is greater than the concentration of H+ ions, the solution is basic and has a pH value greater than 7.

In the case of pure water, the autoionization of water occurs, which is a process where a water molecule splits into H+ and OH- ions. The equilibrium constant for this process is very small, indicating that the concentration of H+ and OH- ions in pure water is very low. At room temperature (25°C), the concentration of H+ and OH- ions in pure water is approximately 1 x 10-7 moles per liter. Therefore, the pH of pure water is very close to 7, which is considered neutral.