Anacidic oxide is an oxide that either produces an acidic solution upon addition to water, or acts as an acceptor ofhydroxide ions effectively functioning as aLewis acid.[1] Acidic oxides will typically have a lowpKa and may beinorganic ororganic. A commonly encountered acidic oxide,carbon dioxide produces an acidic solution (and the generation ofcarbonic acid) when dissolved. Generally non-metallic oxides are acidic.[2]
The acidity of an oxide can be reasonably assumed by its accompanying constituents. Less electronegative elements tend to form basic oxides such assodium oxide andmagnesium oxide, whereas more electronegative elements tend to produce acidic oxides such ascarbon dioxide andphosphorus pentoxide. Some oxides, likealuminium oxides, areamphoteric, while some oxides may be neutral.[3]
Acidic oxides are of environmental concern. Sulfur and nitrogen oxides are considered air pollutants as they react with atmospheric water vapour to produceacid rain.
Carbonic acid is an illustrative example of the Lewis acidity of an acidic oxide.
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.
Aluminium oxide (Al2O3) is anamphoteric oxide; it can act as a base or acid. For example, with base differentaluminate salts will be formed:
Silicon dioxide is an acidic oxide. It will react with strong bases to formsilicate salts.[4]
Silicon dioxide is the anhydride ofsilicic acid:
Phosphorus(III) oxide reacts to formphosphorous acid in water:
Phosphorus(V) oxide reacts with water to givephosphoric acid:
Sulfur dioxide reacts with water to form the weak acid,sulfurous acid:
Sulfur trioxide forms the strong acidsulfuric acid with water:
This reaction is important in the manufacturing of sulfuric acid.
Chlorine(I) oxide reacts with water to formhypochlorous acid, a very weak acid:
Chlorine(VII) oxide reacts with water to formperchloric acid, a strong acid:
Iron(II) oxide is the anhydride of the aqueous ferrous ion:
Chromium trioxide is the anhydride ofchromic acid:
Vanadium trioxide is the anhydride ofvanadous acid:
Vanadium pentoxide is the anhydride ofvanadic acid:
3. Describing a compound that forms an acid when dissolved in water. Carbon dioxide, for example, is an acidic oxide.
