Here's why:
* Dissociation: Water molecules can dissociate into hydrogen ions (H+) and hydroxide ions (OH-) to a very small extent.
* Equilibrium: This dissociation reaches equilibrium, meaning the rate of forward and reverse reactions are equal.
* Equal Concentration: At equilibrium, the concentration of H+ and OH- ions is equal, which is approximately 10^-7 moles per liter.
* pH Scale: The pH scale is logarithmic, where pH 7 represents a neutral solution.
Therefore, distilled water (dm water) is considered neutral with a pH of 7.
Important Note: In reality, distilled water can slightly absorb carbon dioxide from the air, which can make it slightly acidic (pH less than 7). However, for practical purposes, we often assume distilled water to have a pH of 7.
