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Additional recommended knowledge
Acidic oxides as anhydrides
Some acidic oxides react with water to form a well-defined acid. The general equation is
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 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:
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).
|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.|