Iron(III) oxide

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Iron(III) oxide
IUPAC name
Iron(III) oxide
Other names
Colcothar, ferric iron, ferric oxide, hematite, iron sesquioxide, maghemite, ochre, red iron oxide, rouge, rust
Molar mass 159.69 g/mol
Appearance Red solid
Odor Odorless
Density 5.242 g/cm3
Melting point anhydrous
1,539–1,565 °C (2,802–2,849 °F; 1,812–1,838 K) (decomposition)
105 °C (221 °F; 378 K) (decomposition)
150 °C (302 °F; 423 K) (decomposition)
50 °C (122 °F; 323 K) (decomposition)
92 °C (198 °F; 365 K) (decomposition)
Boiling point Decomposes
Solubility Reacts with acids
Hydrates show slight solubility in sugar and mildly acidic aqueous solutions
Insoluble in all organic solvents
87.4 J·mol-1·K-1
−824.2 kJ/mol
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
10,000 mg/kg (rats, oral)
Related compounds
Related compounds
Iron(II,III) oxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Iron(III) oxide, often referred to as red iron oxide, is the chemical compound with chemical formula Fe2O3. It is the main component of rust and acts as the oxidizing agent in the classic thermite reaction with aluminium.



Iron oxide is a typical metal oxide. It will react with acids to form the iron salt, so iron(III) oxide is a useful starting point to make chemicals such iron acetate.

A mixture of red iron oxide and aluminium powders is the classic thermite mixture. This mixture, while difficult to initiate, burns in excess of 1500 °C, producing a slag of red hot molten iron and and aluminium oxide.

Red iron oxide is sometimes added to KNO3 and sugar mixtures to help accelerate the reaction, which is needed when making 'rocket candy' rockets.


Iron(III) oxide is a deep red compound usually encountered in powdered form. It is insoluble in water but readily reacts with acids. It is noticeably ferromagnetic, accumulating thickly on the surface of magnets. While ferrofluid, a magnetic liquid suspension, is typically made with iron(II,III) oxide, the magnetic properties of iron(III) oxide may also allow for this to be made.


Sometimes used as a colouring agent for things such as concrete. Pottery supply stores will have this chemical for the same reason.

Due to the well known thermite reaction, it is commonly available in large amounts online.


Iron can be slowly oxidized by oxygen in the presence of salt water to red iron oxide.

A more effective way to produce large amounts of iron oxide is through electrolysis. A solution of sodium chloride is electrolyzed with two iron electrodes, producing insoluble iron(II) oxide.

This is filtered out and heated until dry and then strong heating in air converts the black iron(II) oxide to the red iron(III) oxide.

A quick way to whip up a lot of this oxide is reacting bulk iron (such as nails) with concentrated (60-70%) nitric acid. The reaction is very vigorous and exothermic, and the nails are quickly converted to iron (III) nitrate (and already a lot of iron (III) oxide, the liquid becomes very brown and opaque). Fuming nitric acid will react very slowly, add water carefully to start the vigorous reaction.

Fe + 6 HNO3(conc) → Fe(NO3)3 + NO2 + 3 H2O (main reaction)
2 Fe + 6 HNO3 → Fe2O3 + 6 NO2 + 3 H2O (useful side reaction)

Add alkali, such as sodium hydroxide, to precipitate more iron (III) hydroxide, which can be calcined to yield red iron oxide. Aqueous ammonia can also be used, as it gives better performance.


  • Make thermite
  • Make elemental iron
  • Ferrofluid, a "liquid magnet"
  • Make magnetite
  • Make ferrates



As an insoluble compound of iron, iron(III) oxide is not substantially toxic unless intentionally swallowed in larger amounts. Obviously, be wary of problems associated with tetanus, mainly if your source of rust came in contact with dirt.

Iron(III) oxide has a tendency to stain objects, especially clothes and is a pain to remove properly.


No special storage is required, though it will stain many materials and clothing and may induce rusting in steels.


No special disposal is required, unless is contaminated with heavy metals.


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