Acidic oxide

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An acidic oxide is an oxide that either produces an acidic solution upon addition to water, or acts as an acceptor of hydroxide ions effectively functioning as a Lewis acid.[1] Acidic oxides will typically have a low pKa and may be inorganic or organic. A commonly encountered acidic oxide, carbon dioxide produces an acidic solution (and the generation of carbonic acid) when dissolved.[2]

The acidity of an oxide can be reasonably assumed by its accompanying constituents. Less electronegative elements tend to form basic oxides such as sodium oxide and magnesium oxide, whereas more electronegative elements tend to produce acidic oxides as seen with carbon dioxide and phosphorus pentoxide. Some oxides like aluminium oxides are amphoteric.[3]

Acidic oxides are of environmental concern. Sulfur and nitrogen oxides are considered air pollutants as they react with atmospheric water vapour to produce acid rain.


Carbonic acid is an illustrative example of the Lewis acidity of an acidic oxide.

CO2 + 2OH ⇌ HCO3 + OH ⇌ CO32− + H2O

This property is a key reason for keeping alkali chemicals well sealed from the atmosphere, as long-term exposure to carbon dioxide in the air can degrade the material.

Further examples[edit]

Aluminium oxide[edit]

Aluminium oxide (Al2O3) is an amphoteric oxide; it can act as a base or acid. For example, with base different aluminate salts will be formed:

Al2O3 + 2 NaOH + 3 H2O → 2 NaAl(OH)4

Silicon dioxide[edit]

Silicon dioxide is an acidic oxide. It will react with strong bases to form silicate salts.[4]

Silicon dioxide is the anhydride of silicic acid:

Phosphorus oxides[edit]

Phosphorus(III) oxide reacts to form phosphorous acid in water:

P4O6 + 6 H2O → 4 H3PO3

Phosphorus(V) oxide reacts with water to give phosphoric acid:

P4O10 + 6 H2O → 4 H3PO4

Sulfur oxides[edit]

Sulfur dioxide reacts with water to form the weak acid, sulfurous acid:

SO2 + H2O → H2SO3

Sulfur trioxide forms the strong acid sulfuric acid with water:

SO3 + H2O → H2SO4

This reaction is important in the manufacturing of sulfuric acid.

Chlorine oxides[edit]

Chlorine(I) oxide reacts with water to form hypochlorous acid, a very weak acid:

Chlorine(VII) oxide reacts with water to form perchloric acid, a strong acid:

Cl2O7 + H2O → 2 HClO4

Iron oxides[edit]

Iron(II) oxide is the anhydride of the aqueous ferrous ion:

Chromium oxides[edit]

Chromium trioxide is the anhydride of chromic acid:

Vanadium oxides[edit]

Vanadium trioxide is the anhydride of vanadous acid:

Vanadium pentoxide is the anhydride of vanadic acid:

See also[edit]


  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  1. ^ John Daintith (February 2008). "acidic". A Dictionary of Chemistry. 3. Describing a compound that forms an acid when dissolved in water. Carbon dioxide, for example, is an acidic oxide.
  2. ^ David Oxtoby; H. P. Gillis; Alan Campion. Principles of Modern Chemistry (7th ed.). Cengage Learning. pp. 675–676. ISBN 978-0-8400-4931-5.
  3. ^ Chang, Raymond; Overby, Jason (2011). General chemistry: the essential concepts (6th ed.). New York, NY: McGraw-Hill. ISBN 9780073375632. OCLC 435711011.
  4. ^ Comprehensive Chemistry Volume 1. 113, Golden House, Daryaganj, New Delhi - 110002, India: Laxmi Publications. 2018. p. 6.13. ISBN 978-81-318-0859-7.{{cite book}}: CS1 maint: location (link)