Acid anhydride A nonmetal oxide that reacts

Acid anhydride A nonmetal oxide that reacts with water to form an acid. 

Chemically, nonmetals are usually the opposite of metals. The nonmetallic nature will increase towards the top of any column and toward the right in any row on the periodic table. Most nonmetal oxides are acid anhydrides. When added to water, they will form acids. A few nonmetals oxides, most notably CO and NO, do not react. Nonmetal oxides that do not react are neutral oxides. The reaction of a nonmetal oxide with water is not an oxidation-reduction reaction. The acid that forms will have the nonmetal in the same oxidation state as in the reacting oxide. The main exception to this is N02, which undergoes an oxidation-reduction (disproportionation) reaction to produce HN03 and NO. When a nonmetal can form more than one oxide, the higher the oxidation number of the nonmetal, the stronger the acid it forms. 

Oxides that react with water to form acids are called acidic anhydrides (anhydride means "without water") or acidic oxides. A few nonmetai oxides, especially ones witii die nonmetal in a low oxidation state— such as N2O, NO, and CO—do not react wifli water and are not acidic anhydrides. 

Many oxides of nonmetals form acidic solutions in water and hence are called acid anhydrides. The familiar laboratory acids HN03 and H2S04, for instance, are derived from acidic binary oxides. Even oxides that do not react with water can be regarded as the formal anhydrides of acids. A formal anhydride of an acid is the molecule obtained by striking out the elements of water (H, H, and O) from the molecular formula of the acid. Carbon monoxide, for instance, is the formal anhydride of formic acid, HCOOH, although CO does not react with cold water to form the acid. 

In Table 5.2.3.5 nonmetal oxides are also considered to be acid anhydrides in some categorization systems since addition of water produces acids. Note also that many nonmetal oxides, like CO2 and SO2, react with water but not in a fashion that would be considered incompatible, since the reaction is not very exothermic or dangerous. For example, exhaling CO2 through a straw into a glass of water does not produce a violent reaction. 

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