Acids, Bases, and Salts Practice Test 2026 - Free Practice Questions and Study Guide

Under Arrhenius theory, bases are substances that increase the concentration of hydroxide ions in water. When a base dissolves, it releases OH− into the solution, making hydroxide the negatively charged ion present. For example, NaOH dissociates into Na+ and OH−, and even ammonia in water forms OH− through NH3 + H2O ⇌ NH4+ + OH−. The other ions listed aren’t produced by dissolving a base in water: H3O− isn’t a real species (hydronium carries a positive charge, H3O+), H− is not stable in aqueous solution, and O2− would rapidly protonate in water to become OH−. So the hydroxide ion is the negative ion produced by a dissolved base.