H2O + H+ + OH- represents the autoionization of water. This is a chemical reaction in which water molecules (H2O) dissociate into hydrogen ions (H+) and hydroxide ions (OH-). This reaction is important because it is responsible for the acidity or basicity of water. The more H+ ions there are in water, the more acidic it is, and the more OH- ions there are in water, the more basic it is.