An oxide is a binary compound that forms when oxygen reacts with another element. All the elements form oxides, except helium, neon, argon, and krypton. Metal oxides are typically ionic crystalline solids (containing the O^2– ion), and are generally bases, though the less electropositive metals form amphoteric oxides with acidic and basic properties (e.g., aluminum oxide). nonmetal oxides are covalent and typically volatile, though a few are macromolecular refractory solids (e.g., silicon dioxide); most are acidic, some are neutral (e.g., carbon monoxide), and water is amphoteric.

 

Oxides may be prepared by direct synthesis, or by heating hydroxides, nitrates, or carbonates. Many oxide minerals are known; simple oxides are binary metal oxides, complex oxides contain several cations, and spinels are intermediate between the two. Some metal oxides (e.g., titanium (II) oxide) are nonstoichiometric.

 

Acidic oxides

If carbon, in for example, coal or coke, is completely burned in air, it combines with oxygen to form carbon dioxide. Carbon dioxide is an acidic oxide. Like sulfur dioxide, it causes atmospheric corrosion because of its acidic properties. If carbon dioxide is dissolved in water, a weak acid, carbonic acid (H₂CO₃) is produced.

 

H₂O + CO₂ → H₂CO₃.

 

In effect, the acidic oxide is acting as if it were an acid and acidic oxides may be considered as anhydrous forms of acid. Looking at this from another point of view, the acid is a hydrated form of oxide.

 

This is well illustrated in the properties of two oxides of sulfur – sulfur dioxide (SO₂) and sulfur trioxide (SO₃). When dissolved in water, sulfur dioxide forms sulfurous acid (H₂SO₃), but on heating, the sulfur dioxide is easily driven off. The formation of acid from oxide and oxide from acid is a reversible process, written as:

 

H₂O + SO₂ ⇌ H₂SO₃

 

Sulfur trioxide, however, reacts violently with and dissolves in water to form sulfuric acid:

 

H₂O + SO₃ → H₂SO₄.

 

This is not a reversible process and the water has to be forcibly extracted, using phosphorus pentoxide, to recover the oxide.

 

2H₂SO₄ + P₄O₁₀ → 2SO₃ + 4HPO₃

 

Sulfur trioxide is, then a true anhydride of sulfuric acid. It is interesting to note that the dehydrating agent, phosphorus pentoxide, is itself an acid oxide forming phosphoric acid with the water molecules removed from the sulfuric acid.

 

Most of the oxides of non-metals are acidic in character – families of acids are produced when, for example, oxides of nitrogen, silicon, and the halogens are dissolved in water.

 

Basic oxides

Whereas the oxides of non-metals tend to be acidic in character, most of the metals form basic oxides. When they combine with water, alkalis are formed. For example, sodium oxide dissolves in water to form sodium hydroxide:

 

Na₂O + H₂O → 2NaOH.

 

Basic oxides are also formed by calcium, copper, and iron, and all form hydroxides with water.

 

Amphoteric oxides

Most oxides are either definitely acidic or basic. But there is an interesting group of oxides that behave as acids in the presence of alkalis, and bases in the presence of acids. These are called amphoteric oxides. They are formed by a number of metals – aluminum, tin, zinc, and antimony among them. Aluminum oxide, for example, forms aluminum chloride in the presence of hydrochloric acid, and thus acts as a base:

 

Al₂O₃ → 2AlCl₃ + 3H₂O.

 

In the presence of sodium hydroxide, however, it acts as an acid, forming sodium aluminate:

 

Al₂O₃ + 2NaOH → 2NaAlO₃ + H₂O.

 

This reaction is used in the extraction of aluminum from its ore, bauxite.