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Acidic oxide

Overview

An acidic oxide (sometimes known as an acidic anhydride, but not to be confused with an acid anhydride) is an oxide

Oxide

An oxide is a chemical compound containing at least one oxygen atom as well as at least one other element. Most of the Earth's crust consists of oxides. Oxides result when elements are oxidized by oxygen in air. Combustion of hydrocarbons affords the two principal oxides of carbon, carbon...

that either

reacts with water
Water
Water is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...

to form an acid
Acid
An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH less than 7.0...

; or
reacts with a base
Base (chemistry)
In chemistry, a base is most commonly thought of as an aqueous substance that can accept hydrogen ions. Bases are also the oxides or hydroxides of metals. A soluble base is also often referred to as an alkali if hydroxide ions are involved. This refers to the Brønsted-Lowry theory of acids and bases...

to form a salt.

Examples include:

Carbon dioxide
Carbon dioxide
Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...

which reacts with water to produce carbonic acid
Carbonic acid
Carbonic acid has the formula H₂CO₃. It is also a name sometimes given to solutions of carbon dioxide in water, which contain small amounts of H₂CO₃. The salts of carbonic acids are called bicarbonates and carbonates. It is a weak acid...

.
Sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula SO₂. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide...

, which does not form the non-existent sulfurous acid
Sulfurous acid
Sulfurous acid is the chemical compound with the formula H₂SO₃. There is no evidence that sulfurous acid exists in solution, but the molecule has been detected in the gas phase...

but does react with bases to form sulfite
Sulfite
Sulfites are compounds that contain the sulfite ion SO . The sulfite ion is the conjugate base of sulfurous acid. Although the acid itself is elusive, its salts are widely used.-Structure:...

s.
Silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with a chemical formula of ' and has been known for its hardness since antiquity. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms...

, which does not react with water but will react with bases to form silicate
Silicate
A silicate is a compound containing an ion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF₆]²⁻, but the silicate species that are encountered most often...

s
Chromium trioxide
Chromium trioxide
Chromium trioxide is the inorganic compound with the formula CrO₃. It is the acid anhydride of chromic acid, and is sometimes marketed under the same name....

, which reacts with water to form chromic acid
Chromic acid
Chromic acid generally refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving of chromium trioxide in sulfuric acid. Often the species are assigned the formulas H₂CrO₄ and...

.

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Encyclopedia

An acidic oxide (sometimes known as an acidic anhydride, but not to be confused with an acid anhydride) is an oxide

Oxide

that either

reacts with water
Water
Water is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...

to form an acid
Acid
An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH less than 7.0...

; or
reacts with a base
Base (chemistry)
In chemistry, a base is most commonly thought of as an aqueous substance that can accept hydrogen ions. Bases are also the oxides or hydroxides of metals. A soluble base is also often referred to as an alkali if hydroxide ions are involved. This refers to the Brønsted-Lowry theory of acids and bases...

to form a salt.

Examples include:

Carbon dioxide
Carbon dioxide
Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...

which reacts with water to produce carbonic acid
Carbonic acid
Carbonic acid has the formula H₂CO₃. It is also a name sometimes given to solutions of carbon dioxide in water, which contain small amounts of H₂CO₃. The salts of carbonic acids are called bicarbonates and carbonates. It is a weak acid...

.
Sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula SO₂. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide...

, which does not form the non-existent sulfurous acid
Sulfurous acid
Sulfurous acid is the chemical compound with the formula H₂SO₃. There is no evidence that sulfurous acid exists in solution, but the molecule has been detected in the gas phase...

but does react with bases to form sulfite
Sulfite
Sulfites are compounds that contain the sulfite ion SO . The sulfite ion is the conjugate base of sulfurous acid. Although the acid itself is elusive, its salts are widely used.-Structure:...

s.
Silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with a chemical formula of ' and has been known for its hardness since antiquity. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms...

, which does not react with water but will react with bases to form silicate
Silicate
A silicate is a compound containing an ion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF₆]²⁻, but the silicate species that are encountered most often...

s
Chromium trioxide
Chromium trioxide
Chromium trioxide is the inorganic compound with the formula CrO₃. It is the acid anhydride of chromic acid, and is sometimes marketed under the same name....

, which reacts with water to form chromic acid
Chromic acid
Chromic acid generally refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving of chromium trioxide in sulfuric acid. Often the species are assigned the formulas H₂CrO₄ and...

. Chromic acid is a hypothetical acid. It would have a structure similar to sulfuric acid.

Acidic oxides are oxides of either nonmetal

Nonmetal

Nonmetal, or non-metal, is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, every element in the periodic table can be termed either a metal or a nonmetal...

s or of metal

Metal

A metal is a chemical element that is a good conductor of both electricity and heat, forms cations and ionic bonds with non-metals. In chemistry, a metal is an element, compound, or alloy characterized by high electrical conductivity. In a metal, atoms readily lose electrons to form positive ions...

s in high oxidation state

Oxidation state

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

Acidic oxides as anhydrides

Some acidic oxides react with water to form a well-defined acid. The general equation is

EO_x + yH₂O → H_2yEO_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

Perrhenic acid

Perrhenic acid is the chemical compound with the formula Re₂O₇₂. It is obtained by evaporating aqueous solutions of Re₂O₇. Conventionally, perrhenic acid is considered to have the formula HReO₄, and a species of this formula forms...

", but the H₂ReO₄ molecule cannot be isolated.

Anhydride	Hydrated form
dichlorine heptoxide Dichlorine heptoxide Dichlorine heptoxide is the chemical compound with the formula Cl₂O₇. This chlorine oxide is the anhydride of perchloric acid. It is produced by the careful distillation of perchloric acid in the presence of the dehydrating agent phosphorus pentoxide:It may also be formed by... , Cl₂O₇	perchloric acid Perchloric acid Perchloric acid, HClO₄, is an oxoacid of chlorine and is a colorless liquid soluble in water. It is a strong acid comparable in strength to sulfuric and nitric acids... , HClO₄
dichlorine pentoxide, Cl₂O₅	chloric acid Chloric acid Chloric acid, HClO₃, is an oxoacid of chlorine, and the formal precursor of chlorate salts. It is a strong acid and oxidizing agent.... , HClO₃
dichlorine trioxide Dichlorine trioxide Dichlorine trioxide, Cl₂O₃, is a chlorine oxide. It is a dark brown solid discovered in 1967 which is explosive even below 0°C.... , Cl₂O₃	chlorous acid Chlorous acid Chlorous acid is a chemical compound with the formula HClO₂. It is a weak acid. Chlorine possesses oxidation state +3 in this acid. The pure substance is unstable, disproportionating to hypochlorous acid and chloric acid .2HClO₂ → HClO + HClO₃Chlorite salts such... , HClO₂
dichlorine monoxide Dichlorine monoxide Dichlorine monoxide, Cl₂O, is a chlorine oxide. It is a brownish yellow gas at room temperature which can explode in high concentrations when heated or sparked.It is best prepared by treating fresh yellow mercuric oxide with Cl₂ gas:... , Cl₂O	hypochlorous acid Hypochlorous acid Hypochlorous acid is a weak acid with the chemical formula HClO. In the swimming pool industry, Hypochlorous acid is referred to as HOCl. It forms when chlorine dissolves in water. It cannot be isolated in pure form due to rapid equilibration with its precursor... , HClO
sulphur trioxide, SO₃	sulphuric acid, H₂SO₄
selenium trioxide Selenium trioxide Selenium trioxide is a chemical compound used in the manufacture of photoelectric cells and solar energy devices.A laboratory preparation is to react anhydrous K₂SeO₄ and SO₃. The compound is thermodynamically unstable with respect to selenium dioxide. The solid... , SeO₃	selenic acid Selenic acid Selenic acid is the chemical compound with the formula . It is an oxoacid of selenium, and its structure is more accurately described as .As predicted by VSEPR theory, the selenium center is tetrahedral, with a Se–O bond length of 161 pm... , H₂SeO₄
selenium dioxide Selenium dioxide Selenium dioxide is the chemical compound with the formula SeO₂. This colorless solid is one of the most frequently encountered compounds of selenium.-Properties:... , SeO₂	selenous acid Selenous acid Selenous acid is the chemical compound with the formula H₂SeO₃. Structurally, it is more accurately described by ₂SeO. It is the principal oxoacid of selenium; the other being selenic acid.... , H₂SeO₃
tellurium trioxide Tellurium trioxide Tellurium trioxide is an inorganic chemical compound of tellurium and oxygen. Tellurium is in the +6 oxidation state. There are two forms, yellow-red α-TeO₃ and grey, rhombohedral, β-TeO₃ which is less reactive. ... , TeO₃	telluric acid Telluric acid Telluric acid is a chemical compound with the formula Te₆. It is a white solid made up of octahedral Te₆ molecules and this structure persists in aqueous solution... , Te(OH)₆
tellurium dioxide Tellurium dioxide Tellurium dioxide is a solid oxide of tellurium. It is encountered in two different forms, the yellow orthorhombic mineral tellurite, β-TeO₂, and the synthetic, colourless tetragonal , α-TeO₂... , TeO₂	tellurous acid Tellurous acid Tellurous acid, H₂TeO₃, is a tellurium oxoacid.It can be obtained as a solid but it is not well characterized.Tellurous acid is not so easily formed by adding tellurium dioxide to water, but it can be made other ways.... , H₂TeO₃
dinitrogen pentoxide Dinitrogen pentoxide Dinitrogen pentoxide is the chemical compound with the formula N₂O₅. Also known as nitrogen pentoxide, N₂O₅ is one of the binary nitrogen oxides, a family of compounds that only contain nitrogen and oxygen... , N₂O₅	nitric acid Nitric acid Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure older samples tend to acquire a stronger yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as... , HNO₃
dinitrogen trioxide Dinitrogen trioxide Dinitrogen trioxide is the chemical compound with the formula N₂O₃. This deep blue liquid is one of binary nitrogen oxides. It forms upon mixing equal parts of nitric oxide and nitrogen dioxide and cooling the mixture below −21 °C :Dinitrogen trioxide is only... , N₂O₃	nitrous acid Nitrous acid Nitrous acid is a weak and monobasic acid known only in solution and in the form of nitrite salts.Nitrous acid is used to make diazides from amines; this occurs by nucleophilic attack of the amine onto the nitrite, reprotonation by the surrounding solvent, and double-elimination of water... , HNO₂
phosphorus pentoxide Phosphorus pentoxide Phosphorus pentoxide is a chemical compound with formula P₂O₅. This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant.-Structure:... , "P₂O₅" i.e. P₄O₁₀	phosphoric acid Phosphoric acid Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H₃PO₄. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more... , H₃PO₄
phosphorus trioxide Phosphorus trioxide Phosphorus trioxide is the chemical compound with the formula P₄O₆. Although it should properly be named tetraphosphorus hexoxide, the name phosphorus trioxide preceded the knowledge of the compound's molecular structure, and its usage continues today. This colorless solid... , "P₂O₃" i.e. P₄O₆	phosphorous acid Phosphorous acid Phosphorous acid is the compound described by the formula H₃PO₃. It can be formulated as HP₂ and therefore contains phosphorus in oxidation state +3. It is one of the oxoacids of phosphorus, other important members being phosphoric acid and hypophosphorous acid... , H₃PO₃
arsenic pentoxide Arsenic pentoxide Arsenic pentoxide, As₂O₅, is an important commercial compound of arsenic. Its usage with respect to arsenic chemistry, as well as its toxicology are similar to arsenic trioxide, As₂O₃... , As₂O₅	arsenic acid Arsenic acid Arsenic acid is the chemical compound with the formula H₃AsO₄. More descriptively written as AsO₃, this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic acid as such has not been isolated, but... , H₃AsO₄
arsenic trioxide Arsenic trioxide Arsenic trioxide is the inorganic compound with the formula As₂O₃. This commercially important oxide of arsenic is the main precursor to other arsenic compounds, including organoarsenic compounds. Approximately 50,000 tons are produced annually... , As₂O₃	arsenous acid Arsenous acid Arsenous acid, also known as arsenious acid, is the hydrolyzed form of arsenic trioxide and has the formula As₃. As₃ occurs in aqueous solutions and has not been isolated as a pure material, although this fact does not detract from the significance of... , H₃AsO₃
carbon dioxide Carbon dioxide Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state... , CO₂	carbonic acid Carbonic acid Carbonic acid has the formula H₂CO₃. It is also a name sometimes given to solutions of carbon dioxide in water, which contain small amounts of H₂CO₃. The salts of carbonic acids are called bicarbonates and carbonates. It is a weak acid... , H₂CO₃
tin dioxide Tin dioxide Tin dioxide is the inorganic compound with the formula SnO₂. The mineral form of SnO₂ is called cassiterite, and this is the main ore of tin. With many other names , this oxide of tin is the most important raw material in tin chemistry... , SnO₂	stannic acid, H₂SnO₃
boron oxide, B₂O₃	boric acid Boric acid Boric acid, also called boracic acid or orthoboric acid or Acidum Boricum, is a weak acid often used as an antiseptic, insecticide, flame retardant, in nuclear power plants to control the fission rate of uranium, and as a precursor of other chemical compounds. It exists in the form of colorless... , H₃BO₃
manganese(VII) oxide Manganese(VII) oxide Manganese oxide is an inorganic compound with the formula Mn₂O₇. This volatile liquid is highly reactive and more often discussed than observed or intentionally prepared. It is a dangerous oxidizer that was first described in 1860... , Mn₂O₇	permanganic acid, HMnO₄
technetium(VII) oxide, Tc₂O₇	pertechnetic acid Pertechnetic acid Technetium oxide is the chemical compound with the formula Tc₂O₇. This yellow volatile solid is a rare example of a molecular binary metal oxide, the other examples being RuO₄, OsO₄, and the unstable Mn₂O₇... , HTcO₄
rhenium(VII) oxide Rhenium(VII) oxide Rhenium oxide is the chemical compound with the formula Re₂O₇. It is the anhydride of the hypothetical acid . Perrhenic acid, Re₂O₇·2H₂O, however is closely related structurally to Re₂O₇... , Re₂O₇	perrhenic acid Perrhenic acid Perrhenic acid is the chemical compound with the formula Re₂O₇₂. It is obtained by evaporating aqueous solutions of Re₂O₇. Conventionally, perrhenic acid is considered to have the formula HReO₄, and a species of this formula forms... , HReO₄

Silicon dioxide

The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with a chemical formula of ' and has been known for its hardness since antiquity. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms...

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

Hydrofluoric acid

Hydrofluoric acid is a solution of hydrogen fluoride in water. While it is extremely corrosive and difficult to handle, it is technically a weak 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

Geochemistry

The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and their...

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

Sodium hydroxide

Sodium hydroxide , also known as lye and caustic soda, is a caustic metallic base. It is used in many industries, mostly as a strong chemical base in the manufacture of pulp and paper, textiles, drinking water, soaps and detergents and as a drain cleaner. Worldwide production in 1998 was around...

to give sodium silicate

Sodium silicate

Sodium silicate is the common name for a compound sodium metasilicate, Na₂SiO₃, also known as water glass or liquid glass. It is available in aqueous solution and in solid form and is used in cements, passive fire protection, refractories, textile and lumber processing, and...

2NaOH + SiO₂ → Na₂SiO₃ + H₂O

At least five different silicic acid

Silicic acid

Silicic acid is a general name for a family of chemical compounds of the element silicon, hydrogen, and oxygen, with the general formula [SiO_x_4-2x]_n...

s are also known, with 0.5–2.5 moles of water per mole of SiO₂ (expressed as SiO₂·nH₂O).