zwitterion(兩性離子)— a species that bears both a positive and a negative charge.Water is the classic example of an amphiprotic solvent, that is, a solvent that can act either as an acid or as a base depending upon the solute. Other common amphiprotic solvents are methanol, ethanol, and anhydrous acetic acid.Amphiprotic solvents undergo self-ionization, or autoprotolysis, to form a pair of ionic species.In a differentiating solvent, various acids dissociate to different degrees and have different strengths. In a leveling solvent, several acids are completely dissociated and show the same strength.