An acidic oxide is an oxide that either produces an acidic solution upon addition to water, or acts as an acceptor of

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...

ions effectively functioning as a Lewis acid. Acidic oxides will typically have a low pKa and may be

inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemist ...

or organic. A commonly encountered acidic oxide,

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

produces an acidic solution (and the generation of carbonic acid) when dissolved. 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 Sodium oxide is a chemical compound with the formula Na2 O. It is used in ceramics and glasses. It is a white solid but the compound is rarely encountered. Instead "sodium oxide" is used to describe components of various materials such as glass ...

and

magnesium oxide Magnesium oxide ( Mg O), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2− ions ...

, whereas more electronegative elements tend to produce acidic oxides as seen with

and

phosphorus pentoxide Phosphorus pentoxide is a chemical compound with molecular formula P4 O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydra ...

. Some oxides like aluminium oxides are

amphoteric In chemistry, an amphoteric compound () is a molecule or ion that can react both as an acid and as a base. What exactly this can mean depends on which definitions of acids and bases are being used. One type of amphoteric species are amphipro ...

. 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.

Examples

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. *

Carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

is also the

anhydride An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the pa ...

of carbonic acid: :H2CO3 -> H2O + CO2 *

Chromium trioxide Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name. This compound is a dark-purple s ...

, which reacts with water forming

chromic acid The term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixtu ...

Dinitrogen pentoxide Dinitrogen pentoxide is the chemical compound with the formula , also known as nitrogen pentoxide or nitric anhydride. It is one of the binary nitrogen oxides, a family of compounds that only contain nitrogen and oxygen. It exists as colourless c ...

, which reacts with water forming

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...

* Manganese heptoxide, which reacts with water forming

permanganic acid Permanganic acid (or manganic(VII) acid) is the inorganic compound with the formula H MnO4. This strong oxoacid has been isolated as its dihydrate. It is the conjugate acid of permanganate salts. It is the subject of few publications and its c ...

Further Examples

Aluminium oxide

Aluminium oxide (Al₂O₃) is an

oxide; it can act as a base or acid. For example, with base different

aluminate In chemistry, an aluminate is a compound containing an oxyanion of aluminium, such as sodium aluminate. In the naming of inorganic compounds, it is a suffix that indicates a polyatomic anion with a central aluminum atom. Aluminate oxyanions Alum ...

salts will be formed: :Al₂O₃ + 2 NaOH + 3 H₂O → 2 NaAl(OH)₄

Silicon dioxide

Silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

is an acidic oxide. It will react with strong bases to form silicate salts.

is the anhydride of

silicic acid Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

: :Si(OH)4 -> 2H2O + SiO2

Phosphorus oxides

Phosphorus(III) oxide Phosphorus trioxide is the chemical compound with the molecular formula P4O6. Although the molecular formula suggests the name tetraphosphorus hexaoxide, the name phosphorus trioxide preceded the knowledge of the compound's molecular structure, a ...

reacts to form

phosphorous acid Phosphorous acid (or phosphonic acid (singular)) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in th ...

in water: :P₄O₆ + 6 H₂O → 4 H₃PO₃

Phosphorus(V) oxide Phosphorus pentoxide is a chemical compound with molecular formula P4 O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydrat ...

reacts with water to give phosphoric (v) acid: :P₄O₁₀ + 6 H₂O → 4 H₃PO₄ Phosphorus trioxide is the anhydride of

: :2H3PO3 -> 3H2O + P2O3

Phosphorus pentoxide Phosphorus pentoxide is a chemical compound with molecular formula P4 O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydra ...

is the anhydride of

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...

: :2H3PO4 -> 3H2O + P2O5

Sulfur oxides

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

Chlorine(I) oxide reacts with water to form

hypochlorous acid Hypochlorous acid (HClO, HOCl, or ClHO) is a weak acid that forms when chlorine dissolves in water, and itself partially dissociates, forming hypochlorite, ClO−. HClO and ClO− are oxidizers, and the primary disinfection agents of chlorine sol ...

, a very weak acid: :Cl2O + H2O <=> 2 HOCl Chlorine(VII) oxide reacts with water to form

perchloric acid Perchloric acid is a mineral acid with the formula H Cl O4. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxidizer when hot, but aqueous s ...

, a strong acid: :Cl₂O₇ + H₂O → 2 HClO₄

Iron oxides

Iron(II) oxide Iron(II) oxide or ferrous oxide is the inorganic compound with the formula FeO. Its mineral form is known as wüstite. One of several iron oxides, it is a black-colored powder that is sometimes confused with rust, the latter of which consists ...

is the anhydride of the aqueous ferrous ion: : e(H2O)62+ -> FeO + 2H+ + 5H2O

Chromium oxides

is the anhydride of

: :H2CrO4 -> H2O + CrO3

Vanadium oxides

Vanadium trioxide is the anhydride of vanadous acid: :2H3VO3 -> 3H2O + V2O3

Vanadium pentoxide Vanadium(V) oxide (''vanadia'') is the inorganic compound with the formula V2 O5. Commonly known as vanadium pentoxide, it is a brown/yellow solid, although when freshly precipitated from aqueous solution, its colour is deep orange. Because o ...

is the anhydride of vanadic acid: :2H3VO4 -> 3H2O + V2O5

References

* {{Reflist

Examples

Further Examples

Aluminium oxide

Silicon dioxide

Phosphorus oxides

Sulfur oxides

Chlorine oxides

Iron oxides

Chromium oxides

Vanadium oxides

See also

References