An acidic oxide is an oxide that either produces an acidic solution upon addition to water, or acts as an acceptor of hydroxide ions effectively functioning as a Lewis acid. Acidic oxides will typically have a low pKa and may be inorganic or organic. A commonly encountered acidic oxide, carbon dioxide produces an acidic solution (and the generation of carbonic acid) when dissolved. The acidity of an oxide can be reasonably assumed by its accompanying constituents. Less electronegative elements tend to form basic oxides such as sodium oxide and magnesium oxide, whereas more electronegative elements tend to produce acidic oxides as seen with carbon dioxide and phosphorus pentoxide. Some oxides like aluminium oxides are amphoteric. Acidic oxides are of environmental concern. Sulfur and nitrogen oxides are considered air pollutants as they react with atmospheric water vapour to produce acid rain. Examples Carbonic acid is an illustrative example of the Lewis acidity of an acidic oxide. CO2 + 2OH− HCO3− + OH− CO32− + H2O This property is a key reason for keeping alkali chemicals well sealed from the atmosphere, as long-term exposure to carbon dioxide in the air can degrade the material. Carbon dioxide is also the anhydride of carbonic acid: H2CO3 -> H2O + CO2 Chromium trioxide, which reacts with water forming chromic acid Dinitrogen pentoxide, which reacts with water forming nitric acid Manganese heptoxide, which reacts with water forming permanganic acid Further examples Aluminium oxide Aluminium oxide (Al2O3) is an amphoteric oxide; it can act as a base or acid. For example, with base different aluminate salts will be formed: Al2O3 + 2 NaOH + 3 H2O → 2 NaAl(OH)4 Silicon dioxide Silicon dioxide is an acidic oxide. It will react with strong bases to form silicate salts. Silicon dioxide is the anhydride of silicic acid: Si(OH)4 -> 2H2O + SiO2 Phosphorus oxides Phosphorus(III) oxide reacts to form phosphorous acid in water: P4O6 + 6 H2O → 4 H3PO3 Phosphorus(V) oxide reacts with water to give phosphoric (v) acid: P4O10 + 6 H2O → 4 H3PO4 Phosphorus trioxide is the anhydride of phosphorous acid: 2H3PO3 -> 3H2O + P2O3 Phosphorus pentoxide is the anhydride of phosphoric acid: 2H3PO4 -> 3H2O + P2O5 Sulfur oxides Sulfur dioxide reacts with water to form the weak acid, sulfurous acid: SO2 + H2O → H2SO3 Sulfur trioxide forms the strong acid sulfuric acid with water: SO3 + H2O → H2SO4 This reaction