Iron(III) oxide

Iron(III) oxide, also called ferric oxide, is a chemical compound with the formula Fe₂O₃. It is found in nature as a mineral called hematite, which is very important for getting iron to make steel. Because of its red color, it is often used as a pigment in things like paint and dyes.

It is one of the three main oxides of iron. The other two are iron(II) oxide, which is not found very often, and iron(II,III) oxide, which occurs naturally as the mineral magnetite.

People sometimes call iron(III) oxide "rust" because it looks similar, but in science, rust is a more complex material that is not exactly the same. Iron(III) oxide can be broken down by weak acids. It also can act as a weak oxidising agent, especially when it reacts with aluminium in a very exciting reaction known as the thermite reaction.

Structure

Fe₂O₃ can exist in different forms, called polymorphs. In one common form, each iron atom is connected to six oxygen atoms. In another form, some iron atoms are connected to four oxygen atoms.

The most common form of Fe₂O₃ is found in nature as a mineral called hematite, which is used to get iron for making steel. This form has a special crystal shape and has interesting magnetic properties.

Another form, called maghemite, has a cubic shape and is used in things like recording tapes. There are also other less common forms of Fe₂O₃ that scientists are studying.

Hydrated iron(III) oxides

There are several forms of hydrated iron(III) oxide. When certain substances are added to iron(III) salts in water, a red-brown substance forms. This is not iron(III) hydroxide, but a compound written as Fe₂O₃·H₂O or Fe(O)OH.

Two common forms of hydrated iron oxide are lepidocrocite, which is red and found on the outside of rust structures, and goethite, which is orange and found inside them. When heated, Fe₂O₃·H₂O loses its water. With more heating, it changes into a black compound called magnetite.

Fe(O)OH can dissolve in acids, and in strong alkaline solutions, it forms certain iron compounds.

Reactions

Iron(III) oxide has important reactions that help make steel and other materials. One key reaction uses carbon monoxide to produce iron from the oxide, which is essential for steel-making.

Another exciting reaction is with aluminium, called the thermite reaction. This creates intense heat and produces molten iron, useful for welding train tracks and creating small iron tools. Iron(III) oxide can also react with hydrogen to form magnetite, a magnetic material. It dissolves in strong acids and certain special solutions too.

Preparation

Iron(III) oxide is formed when iron is exposed to oxygen. In a lab, it can be made by passing an electric current through a solution containing sodium bicarbonate and an iron electrode. This creates a hydrated form of iron(III) oxide, which then turns into iron(III) oxide when heated.

Uses

Iron(III) oxide is mainly used in making steel and iron. It has been used since the Middle Ages in colored glass to create yellow, orange, and red shades.

A very fine powder of this compound, called jeweler’s rouge, helps polish jewelry and lenses. It is also used in some cosmetics and as a coloring material, known by names like Pigment Brown 6 and Pigment Red 101. In the past, it was the main material for magnetic tapes and disks before newer technologies took over. It can also help in treating mild itchiness, as part of calamine lotion.