Acidic oxide


 * This article is about inorganic acidic anhydrides. For the organic functional group, see acid anhydride.

An acidic oxide (sometimes known as an acidic anhydride, but not to be confused with an acid anhydride) is an oxide that either
 * reacts with water to form an acid; or
 * reacts with a base to form a salt.

Examples include:
 * Carbon dioxide which reacts with water to produce carbonic acid.
 * Sulfur dioxide, which does not form the non-existent sulfurous acid but does react with bases to form sulfites.
 * Silicon dioxide, which does not react with water but will react with bases to form silicates
 * Chromium trioxide, which reacts with water to form chromic acid.

Acidic oxides are oxides of either nonmetals or of metals in high oxidation states.

Acidic oxides as anhydrides
Some acidic oxides react with water to form a well-defined acid. The general equation is
 * EOx + yH2O &rarr; H2yEOx+y

although the exact stoichiometry varies from case to case.Sometimes the acid is only known in solution: for example, orange rhenium heptoxide dissolves in water to give a colorless, acidic solution containing perrhenate ions, known as "perrhenic acid", but the H2ReO4 molecule cannot be isolated.

Silicon dioxide
Silicon dioxide is sometimes said to be a special case, in showing no reactivity at all towards water or aqueous acids or bases (with the exception of hydrofluoric acid). In fact, it will dissolve slowly in hot concentrated aqueous alkali, and will hydrate at high temperatures and pressures (a reaction of great geochemical importance). The slow etching of glass (which is "impure" silica) by aqueous alkalis is of considerable practical importance in chemical laboratories. Silicon dioxide can be best seen to be an acidic oxide, in common with the other dioxides of group 14 by its reaction with molten sodium hydroxide to give sodium silicate:


 * 2NaOH + SiO2 → Na2SiO3 + H2O

At least five different silicic acids are also known, with 0.5–2.5 moles of water per mole of SiO2 (expressed as SiO2·nH2O).