Most people think of acids and bases as opposites, but what is opposite about them? The key to understanding acids and bases is water. Water ionizes to produce a small but equal amount of hydronium and hydroxide ions. Addition of acid causes the hydronium concentration to increase while hydroxide concentration decreases. Addition of base causes the hydroxide ion concentration to increase while the hydronium ion concentration decreases. The hydronium ion causes the acid's properties. The hydroxide ion causes the bases's properties. This PowerPoint presentation defines acids and bases based on their effect on the ions in water and describes their properties. Worksheets on this topic are available free of charge on my website at www.evanschemistrycorner.com.