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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.

Contents

Acidic oxides as anhydrides

Silicon dioxide

See also

References

Acidic oxides as anhydrides

Some acidic oxides react with water to form a well-defined acid. The general equation is
::EO_''x'' + ''y''H₂O → H_2''y''EO_''x''+''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.

Oxide	Hydrated acid
dichlorine heptoxide, Cl2O7	perchloric acid, HClO4
dichlorine pentoxide, Cl2O5	chloric acid, HClO3
dichlorine trioxide, Cl2O3	chlorous acid, HClO2
dichlorine monoxide, Cl2O	hypochlorous acid, HClO
sulfur trioxide, SO3	sulfuric acid, H2SO4
selenium trioxide, SeO3	selenic acid, H2SeO4
selenium dioxide, SeO2	selenous acid, H2SeO3
tellurium trioxide, TeO3	telluric acid, Te(OH)6
tellurium dioxide, TeO2	tellurous acid, H2TeO3
dinitrogen pentoxide, N2O5	nitric acid, HNO3
dinitrogen trioxide, N2O3	nitrous acid, HNO2
phosphorus pentoxide, "P2O5" i.e. P4O10	phosphoric acid, H3PO4
phosphorus trioxide, "P2O3" i.e. P4O6	phosphorous acid, H3PO3
arsenic pentoxide, As2O5	arsenic acid, H3AsO4
arsenic trioxide, As2O3	arsenous acid, H3AsO3
carbon dioxide, CO2	carbonic acid, H2CO3
tin dioxide, SnO2	stannic acid, H2SnO3
boron oxide, B2O3	boric acid, H3BO3
manganese(VII) oxide, Mn2O7	permanganic acid, HMnO4
technetium(VII) oxide, Tc2O7	pertechnetic acid, HTcO4
rhenium(VII) oxide, Re2O7	perrhenic acid, HReO4

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·''n''H₂O).

See also

★ Basic oxide

★ Amphoteric oxide

★ Neutral oxide

References

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