1. Acids donate hydrogen ions in water, forming hydronium ions, and have characteristics like turning litmus paper red and tasting sour. Bases form hydroxide ions in water and have opposite characteristics. 2. The pH scale measures how acidic or basic a substance is based on its hydronium ion concentration, with values from 0-14. More acidic substances have fewer hydronium ions and lower pH values closer to 0. 3. A substance is neutral when it has equal concentrations of hydronium and hydroxide ions, resulting in a pH of 7.