Anhydride means without water. Imagine chemical formulas were like numbers and simple subtractions could be done. Interestingly if water is subtracted from an oxyacid, the remainder is a nonmetallic oxide. If water is subtracted from a base, the remainder is a metallic oxide. Nonmetallic oxides such as carbon dioxide are responsible for acid rain. Metallic oxides such as lime, or calcium oxide, are added to soil to counteract acidity. This PowerPoint presentation defines anhydrides and helps students to understand why nonmetallic oxides are acid anhydrides and metallic oxides are base anhydrides. Worksheets on this topic are available free of charge on my website at www.evanschemistrycorner.com.