- Acidic oxide
An acidic oxide (sometimes known as an acidic anhydride, but not to be confused with an
acid anhydride ) is anoxide that either
*reacts withwater to form anacid ; or
*reacts with a base to form a salt.Examples include:
*Carbon dioxide which reacts with water to producecarbonic acid .
*Sulfur dioxide , which does not form the non-existentsulfurous acid but does react with bases to formsulfite s.
*Silicon dioxide , which does not react with water but will react with bases to formsilicate s
*Chromium trioxide , which reacts with water to formchromic acid .Acidic oxides are oxides of either
nonmetal s or ofmetal s in highoxidation state s.Acidic oxides as anhydrides
Some acidic oxides react with water to form a well-defined acid. The general equation is::EO"x" + "y"H2O → H2"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.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 ofhydrofluoric 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 ofgroup 14 by its reaction with moltensodium hydroxide to givesodium silicate :::2NaOH + SiO2 → Na2SiO3 + H2O
At least five different
silicic acid s 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
*Greenwood&Earnshaw
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