There is more to an acid than a sour taste and a low pH. There are three models of acids and bases: The Arrhenius Theory; the Brönsted-Lowry Model; and the Lewis Model. This PowerPoint presentation explains what these models are and how they are related to each other. The nature of conjugate acids and bases is explored, and amphoteric substances are described. The combination of a Lewis acid and base is illustrated. Worksheets on this topic are available free of charge on my website at www.evanschemistrycorner.com.