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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:

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 → 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.

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·nH2O).

See also


  • Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Acidic_oxide". A list of authors is available in Wikipedia.
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